Avoid a precedent
Especially when looking at the expansion of China, this would make a precedent and motivation to further expand if it was not clearly dismissed by the world.
Comparability with ethnical land expansion
It is the comparability with history of expansionist countries claiming for land and and winning that makes this so dangerous.
Prussia took Schleswig-Holstein from the Danes, 50.000 Danes (of which also some Schleswig-Holstein Germans that fought on the Danish side) had to die, and the reasoning of Prussia was ethnical. The Schleswig area south of Denmark had been mainly ethnically Danish long before, but with poverty and famine, Schleswig Danes had to sell their land to Holsteiners. Schleswig-Holstein itself was Danish for 400 years, though Holstein was (Low-)German Speaking. The Holsteiners took over Schleswig land in a poverty phase of the Danes, it was a buyout of land. When Denmark tried to keep the territory even though there was now follower on the throne, and when it tried to make Danish the administrative language everywhere, Prussia took this as a chance to wage war and get the land. It enforced High-German as administrative language in the whole northern Germany to centralise the West-Germanic Low- and High-German speaking Germans so that the "German" borders were strengthened, thereby successfully taking away a good share of the local identity of the north and that of Schleswig-Holstein (Danish would not have done that, it would have rather worked the other way round). In the end, the settlements decided the history. When after this Prussian annexation in 1867, there was a democratic and therefore seemingly justified vote on the borders many decades later in 1920, the border of the final people's vote mirrored the ethnical settlement border (see https://en.wikipedia.org/wiki/Province_of_Schleswig-Holstein):
Following the defeat of Imperial Germany in World War I, the Allied powers organised two plebiscites in Northern and Central Schleswig on 10 February and 14 March 1920, respectively. In Northern Schleswig, 75% voted for reunification with Denmark and 25% for staying with Germany. In Central Schleswig, the situation was reversed, with 80% voting for Germany and 20% for Denmark.
Nazi-Germany annexed the Czech Republic for its expansion and had the plan to assimilate the population and send into the east those who should not be part of it. This would have been done if the war was not lost.
Turkey pushed back and committed genocide on Armenians to get their land during WW1. In 2016, a Turkish general called Armenia a historical mistake and that the land was actually Turkish, see https://en.wikipedia.org/wiki/Revanchism#Turkey, although Turks have conquered the area in the time of the cruzades while Armenia exists since at least 782 B.C., with the capital Yerevan as one of the world's oldest continuously inhabited cities, and evidence of civilization in Armenian area dating to about 4000 BC, see https://en.wikipedia.org/wiki/Armenia#Antiquity.
From their homelands near the Aral sea, the Seljuqs advanced first into Khorasan and then into mainland Persia before eventually conquering eastern Anatolia.  The Seljuq/Seljuk empire was founded by Tughril Beg (1016-1063) in 1037.
Azerbaijan is still at war with Armenia because of land conflicts. Officially, Armenians are said to occupy their land, which is historically wrong since Turks conquered the land long ago, but politically right in the borders of today, see https://en.wikipedia.org/wiki/Azerbaijan#Antiquity.
In Azerbaijan, there is also a minority of Udines as the descendants of the Causcasian Albania, of which those who are still Christians can sometimes speak the old language even today. They have minority rights, but no land. Admittedly, this story goes back more than 1000 years, and Caucasian Albania was already occupied by Iranian Sassinids before, but it shows how land of a minor power can be lost to a stronger power.
Kievan Rus took western Khazar Khaganate land which is now eastern Ukraine. The Khazars do not exist anymore. See https://www.youtube.com/watch?v=Xr2CtmpKU6A, see 18:37, most battles around 900 to 970 A.D. (One could say from this that the eastern Ukraine nowadays is historically neither Russian nor Ukrainian (Kievan Rus is the founding ideology of Russia, White Russia and Ukraine altogether). On the other hand, also the Khazars had expanded before. Unclear point, but worth some more research)
Northern Ireland is just a conflict arising from British expansion to Ireland.
Japan took Hokkaido, the former land of the Ainu, mainly by migration. It also was trading and military power that made it possible.
Mass migration by the Han Chinese took the Manchuria.
Many small peoples and tribes were pushed away by bigger peoples over the thousands of years, and nowadays, the ideology has changed: small peoples have the same cultural rights to exist as big peoples, the world community is different.
The Crimea occupation, the conflicts of Azerbaijan (supported by Turkey) with Armenia and the expansion of China are recent historical struggles for land. With this background, the world was already alerted. But this Ukrainian war is a precedent for the world. It is not just since the NATO got back an old aggressor again or since it is war in Europe, but it is since it bears learning effects for the future history of expansionist countries.
Reduced or misunderstood ethnical border conflict
The border conflict might be misunderstood by nations that also have a border conflict and think it is just about the border. In the Ukrainian case, it is about Russian mass immigration since Bolshevik times, and the border conflict only comes from that.
This is not comparable with the artificial border lines in Africa taken over from colonial times. This speech against expanding across the neighbor border to unify with the same brethren does not mention the problem of the organised ethnical Russian invasion that had happened before but is still highly praised and said to fully address the war: Ukraine: Kenyan ambassador's incredible speech to UN (which is not understanding the problem but gets very strong feedback from everywhere). I hope that the war in Ukraine is not just reduced by the United Nations to a border conflict to unite Russians on both sides? Even though the speech clearly stands against expansionism of course, which is the good core of it all, it does not mention the invasion of the past, it seems as if it sees only the war of today as the expansionist deed. And that would not understand that the absence of such land-taking migration actually avoids the problem.
From such reductions and misunderstandings one can see that the war also reaches out to any ethnical border conflict in the world, and there is so many of them that this war in Ukraine finds so many nations that care for the situation. For a good reason, of course, but also with some share of misunderstandings, it seems.
Side-note: Instead of Irredentism, have a look at Revanchism to see a longer list of what is already listed at the beginning of the answer. Mind that some claims might not go back long enough. For example, in the case of Germany, it might be forgotten that the coastline was also settled by Germans after the cruzades into these areas. I do not know enough about it, but if you go for revanchism, you also must be fair and go back even further in time if needed. That could even mean for the Danes that Schleswig-Holstein was not Danish in the end, since the Vikings took the land and pushed away West Germanic tribes (Vikings were just Northern Germanic tribes) even from more Northern Jutland much longer ago. It is therefore also a balance act how long you go back in time. Revanchism is not irredentism and not mere expansionism, and it should not be condemned, but get more empathy. It shows historically unjust ethnical expansion that was wrong then and therefore is still wrong, it shows ethnical crime of the past that should not be forgotten. Mind that revanchism can also be too short-sighted even if it sounded right at first; it must be seen from a very long historical viewpoint, going back in time through all known steps you can find, and then be judged upon, not just by the recent centuries.
On top of this, and most relevant in my eyes, the Bolsheviks used the Holodomor to weaken Ukraine and then settled the thinned out areas in the east with White Russians and Russians. Since Holodomor was all over the rural areas, one might say that the Holodomor is not directly connected to these settlements. But since Holodomor weakened the whole people, there was also no pressure of Ukrainians to settle the area again or fight the settlement. It was an official punishment by Stalin for the too low harvests of the foregoing years which Stalin saw as a mistrust against the central power. In the centuries before the Holodomor, Ukrainian cossacks had both fought for and against Russia, therefore the mistrust against Ukrainian cossacks was historically grounded. As a hint at the dimension, the Holodomor is of such a scale that it is not accepted as genocide by Israel since it might relativate the Holocaust. Often, Ukrainian cossacks in the rural areas could not flee when they "were starved" to death: 16% of the rural and 4% of the urban population. These numbers show only the death rate, I do not know how many fled into the urban areas, which is an important point: the rural area was likely thinned out much more than 16%. The Holodomor was also in the east of Ukraine where Russian cossacks and also some Ukrainian cossacks had settled before in agreement with Russia. More on cossacks at Can the aggressions of Russia against Ukrainian territory be understood in a politico-historical light? [closed] with a further link to the long history of those cossacks that likely had to starve for historical reasons and had to flee from Russia already in earlier times.
I added Q/As on the ethnical dimension of the Russian expansion on Holodomor and the Ukrainian Cossacks that lived in the rural Ukraine, they were closed, so do not take this as a professional list.
This list is of course too small to reach the core of all this. It is just a hint. Fore more, see this overview of the university of Minnessota on the Holdomor With a link to the documentary "Harvest of Despair".
The Holodomor plays a part in why this land is not left to the Russians for nothing, since the land of their people had already unjustly been settled by non-Ukrainian Soviets before. Before 1934, there were only Ukrainians, and any Russian mother tongue speaker is an immigrant in the area. This event throws a new light on any ongoing or future homogenous mass immigration that creates new majorities and then might finally take over the land.
Immigration plan from 90 years ago
There might even be a long-term plan, not public but at least known to Putin and which should also not astonish anyone who has been called to settle there, a plan that comes already from Stalin times, with an aim to reach the Black Sea and suppress Ukraine. If that was true, this war is not a problem of leadership (Putin), but a systemic problem that only Russia as a whole is responsible for. If that was true, one could also guess that any actions in the past were already a share of the plan. I recall for example the huge sponsorship of Gazprom for a German football team which could not pay off economically since there were no direct clients in Germany. Or on the opposite, the bad deeds like assassinations up to sending off diplomats just when everyone was invited, these deeds were then only to cool down the relationships so that war would not be a shock of disappointment. If you look at the separatist actions, there are clearly many Russian speakers involved in a kind of plan, not just Putin.
The corridor, a map from The Russian ‘invasion’ of Ukraine in maps and numbers - Map: Areas of Ukraine where there has been anti-government or pro-Russia separatist movements March-August 2014 with areas of pro-Russian separatist movements:
If the "Ukrainian" Russians themselves behave like it, they perhaps knew about the long-term plan - from the start. No one would say it of course, but it looks like a long-term Russian conspiracy that was covered by each nowadays "Ukrainian" Russians, without needed public debate on it so that no one would have a proof. It would fit in the ideology of that time of expansion of the strong peoples to push away the weak peoples to the side.
Already the historical events before the Crimea occupation look as if to fulfill a Stalinist and Bolshevik long-term plan of Russian expansion to the Black Sea.
But the events of Crimea and eastern Ukraine might show this plan more clearly and hint at the Holodomor (which hit mainly the rural areas which were then settled, not sure whether only in the east, though) to have been a calculated deed for geostrategic aims from the start since it seems like a tool, it did not just happen for nothing. All of this can be seen from the mere Russian behaviour and wrong historical reasoning of today. And therefore, such political events can change the way the overall history can be seen.
This answer in short
Avoid a precedent that teaches expansionist countries "how it goes".
There will be so strong bad spillovers of this conflict for the world that even China, which likes to expand, is against it (UPDATE: it has shifted towards being just neutral), since the nations need unity to get over this, else, they will all lose more than they gain. This conflict influences future history, the world community knows that the world will learn from it - in a good or bad way is in the hands of the nations themselves. For these massive spillover effects on any ethnical expansion of the future, this war is different from the other wars listed in the question.