I noticed that the 4th largest "party" in the current Dáil are independents with 19/160 seats. Is this proportion high (or not) in a broader European perspective in national legislatures?
Yes. Below is a boxplot which shows the current (March 2020) proportion of independent members of each house of each national parliament in the European Union. From this, we can identify outliers in a statistical sense - any chamber that has more than ~6% of its membership represented by independent candidates should be considered an outlier.
The chambers that match these criteria in descending order are both chambers of the Irish Parliament; the Senead and the Dáil, the Romanian Senate, the Croatian Parliament, and the Latvian Saeima. The full figures used can be found below, and were pulled from Wikipedia. Note that I have excluded the Slovenian National Council, as the membership represents interest groups in the country and is not directly elected, it doesn't make sense to include it in this data.
Full data used:
|Romania||Chamber of Deputies||329||19||5.78%|
|Italy||Senate of the Republic||321||9||2.80%|
|Italy||Chamber of Deputies||630||15||2.38%|
|Cyprus||House of Representatives||56||1||1.79%|
|The Netherland||House of Representatives||150||1||0.67%|
|Portugal||Assembly of the Republic||230||1||0.43%|
|Belgium||Chamber of Representatives||150||0||0.00%|
|Czech Republic||Chamber of Deputies||200||0||0.00%|
|Luxembourg||Chamber of Deputies||60||0||0.00%|
|Spain||Congress of Deputies||350||0||0.00%|