Meh, the more obvious reason, according to the BBC:
Ireland made clear it could not join the North Atlantic Treaty Organisation (Nato) after World War Two because of partition - Northern Ireland remains part of the UK.
Seemingly, Fine Gael is the more pro-Nato party
There are also differences of opinion on military neutrality in the three coalition government parties.
Fine Gael has historically been more questioning of the policy; the Greens strongly supportive, as are most in Fianna Fáil. [...]
Polls show that younger people and supporters of left-wing parties including Sinn Féin, the main opposition party, overwhelmingly support neutrality.
Regarding the 1s issue and @phoog's comment: when NATO was formed at least, that point was made explicit:
Ireland did not join NATO not because of disagreement with its objectives but because
of partition. This was clearly set out by Sean MacBride, the Minister for External Affairs, in
his remarks to the Daiil on 23 February 1949:
... Ireland, as an essentially democratic and freedom-loving country is anxious to play her
full part in protecting and preserving Christian civilisation and the democratic way of life.
With the general aim of the proposed Atlantic Pact in this regard, therefore, we are in
agreement. In the matter of military measures, however, we are faced with an insuperable
difficulty, from the strategic and political points of view, by reason of the fact that six of our
north-eastern counties are occupied by British forces against the will of the overwhelming
majority of the Irish people. Partition is naturally and bitterly resented by the people of this
country as a violation of Ireland's territorial integrity and a denial in her case of the
elementary democratic right of national self-determination. As long as Partition lasts, any alliance or commitment involving joint military action with the State responsible military for Partition must be quite out of the question so far as Ireland is concerned.
Later governments have probably put it in more diplomatic terms (I didn't read the whole paper.)
Apparently, at one point, Irish leaders were considering joining NATO too if that were a prerequisite for joining the EEC.
In an official note written on January 20th, 1962, for then taoiseach Seán Lemass, Whitaker, the leading official involved in the talks on EEC membership, put on record a conversation he had had at lunch with Albert Borschette, the influential Luxembourg permanent representative to the EEC who went on to become a European Commissioner.
Borschette asked Whitaker if the difficulty Ireland had with Nato, which had been referred to that day in a statement by the taoiseach to the Council of Ministers would still apply if Nato, as he thought likely, was to become the main instrument of the common defence policy of the EEC.
“I said that the taoiseach had not entered any such reservation in his statement which was a positive declaration of full acceptance by Ireland, as a member of the community, of all the political implications of membership . . . We recognised, of course, that a common foreign policy and a common defence policy were implicit in the ideal of European unity and the Irish government had declared its full adherence to this ideal.”
In response, Borschette agreed that no definite provisions regarding common defence had yet been worked out by the committee looking into the issue but he thought it likely that, in order to avoid duplication, use would be made of Nato as an organ of that policy. He asked if this would be a stumbling block to the Irish government.
“I replied that, if Ireland was a member of the community and the community decided that Nato should be used as an instrument of a common defence policy, I felt sure the Irish government would, in the new circumstances, be prepared to reconsider their position in relation to Nato.
But since this hypothetical prerequisite didn't materialize as such, the Irish government didn't have to make that choice. In a speech that taoiseach Lemass had given a few days before, he reiterated (with some nuance) the position that Ireland had before:
The conversation took place after Lemass in his statement to the Council of Ministers in Luxembourg seeking support for this country’s application said that while Ireland did not accede to the North Atlantic Treaty, it had always agreed to its general aims. “The fact that we did not accede to it was due to special circumstances (partition) and does not qualify in any way our acceptance of the ideal of European unity and of the conception embodied in the Treaty of Rome and the Bonn Declaration of 18 July last of the duties, obligations and responsibilities which European unity would impose.”
So it seems clear enough that if Irish leadership were willing to accept NATO membership as price of admission in the EEC (then EU) the probably saw article 42.7 as a similarly reasonable compromise (although I don't know the specifics of discussions around that issue, if there were any.)
More recently, RTE noted:
"Ireland is a neutral country, we're militarily non-aligned, but we are certainly not neutral on an issue like this, when there is blatant aggression happening on the continent of Europe."
The words of Simon Coveney, the Minister for Foreign Affairs, speaking on RTÉ's Morning Ireland in the hours after Russia launched its invasion of Ukraine.
Neutral - but not neutral.
Some eyebrows might have been raised at this formulation, but in fact, it fits into Ireland's long tradition of nuanced neutrality.
[...]
Taoiseach Seán Lemass made no bones about his view. In September 1962, he told journalists: "We do not wish in the conflict between the free democracies and the Communist empires to be thought of as neutral. We are not neutral and do not wish to be regarded as such."
His successor, Jack Lynch, told the Dáil in July 1969 that if Ireland were admitted to the EEC, "we would naturally be interested in the defence of the territories embraced by the Communities. There is no question of neutrality there."
Of course, Ireland is no longer the only neutral in the European Union, having been joined by Sweden, Finland and Austria, and moves towards a common defence remain controversial.
Even the EU itself raised similar concerns towards other countries, but these were essentially papered over, one way or another, e.g.
When Austria applied to join the European Communities (EC) in 1989, the European Commission opined that the applicant’s “permanent neutrality” would be incompatible “with the provisions of the existing treaties” and would pose a problem to “the obligations entailed [by the EC’s] future common foreign and security policy”. Still, in 1993, shortly after the Maastricht Treaty converted the EC into the European Union and established the Common Foreign and Security Policy, the bloc launched accession negotiations with neutral Austria. The country became a member of the EU in 1995.
Austria is not the only state that has joined the EU or its predecessor organisations despite being neutral. Ireland joined the EC in 1973. Finland and Sweden became members of the EU in 1995; Cyprus and Malta in 2004. These six countries self-identify as “neutral” or “non-aligned”. Some of them have enshrined this status in their constitutions; others’ neutrality is less formalised but no less ingrained. While neutrality remains a somewhat elusive and ambiguous concept in international relations, experts generally agree that neutral states are prohibited from joining military alliances. This is why none of the six neutral states is a member of NATO. They are, however, members of the EU – whose declared aim is to become a “fully-fledged European defence union” by 2025.
Apparently, this was less of an issue with Ireland (than with Austria) because:
Ireland’s neutrality is not enshrined in its constitution or laws, or in any international treaty. The country’s 2020 Programme for Government sets out a policy of “active military neutrality” that allows for the continuation of both a flexible and participative multilateral approach.
The practicalities of this form neutrality were broiled down to the "triple lock" for Irish force deployments:
Neutrality is no obstacle to Ireland broadly participating in the CSDP. The country has been involved in several EU defence initiatives, including Permanent Structured Cooperation (PESCO), the Coordinated Annual Review of Defence (CARD), the European Defence Fund (EDF), and military and civilian missions led by the bloc. Today, all Irish decision-making on the CSDP turns on a so-called ‘triple lock’ mechanism. In line with the Defence Acts, the Irish Defence Forces cannot be deployed to any conflict zone or CSDP mission without the approval of the UN, the government, and the Dail, the lower house of parliament. [...]
And not sending lethal aid (but not blocking it either):
As an off-budget mechanism, the European Peace Facility allows the EU to provide assistance to CSDP missions and operations. In negotiations to set up the facility, Ireland and like-minded states ensured that it included a safeguard to “constructively abstain” from assistance measures that involve lethal equipment. This approach reflected Ireland’s interests and the 2020 Programme for Government, which stated that Ireland “will not be part of decision-making or funding for lethal force weapons for non-peacekeeping purposes”.
But 42.7 was actually a big deal in Ireland in term of Irish voters' opposition, essentially requiring a special form of opt-out agreed with the EU:
From an Irish perspective, Article 42.7 of the Lisbon Treaty did not turn the EU into a defence union as it avoided the development of a “common defence”. While the EU mutual assistance clause allows member states to provide “aid and assistance” in the case of an attack, it is intergovernmental and involves bilateral consultations with different states. Ireland, Finland, and Sweden can preserve their neutrality or military non-alignment in this framework.
In an Irish referendum on the treaty in June 2008, 53.4 per cent of participants voted ‘no’ after campaigners argued that that it could undermine Ireland’s military neutrality, making reference to Article 42.7. The European Council adopted in July 2009 a decision that provided guarantees to Ireland on defence matters. They included the discretion for countries with a traditional policy of neutrality “to determine the nature of aid or assistance” in the case of armed aggression on the territory of another member state. Another safeguard for Ireland included a requirement for a unanimous decision of the European Council for “any decision to move to a common defence”.
I'm less familiar with some of the agreements mentioned in the question, but even Russia joined the PfP, so that probably doesn't mean much. Also EAPC seems to include all post-Soviet republics except Russia, but including e.g. Belarus. So YMMV as to relevance.
