The best way to do this is to base this assumption on the size of the Swedish armed forces, Army which had conscription, Airforce and Navy.
For example, the Royal Swedish Airforce had 400 combat aircraft at the height of the Cold War.
Then divide their population by Ireland's then you've got a rough estimation of the size of Eire's armed forces every year from 1920 to the present.
WWW.POPULSTAT is a good website for populations of countries per year.
or you can just base it on the Israeli Armed Forces with has mandatory conscription for men/women over the age of 18.
Population in 1990 R.O.I 3.5 million
ISRAEL 2.5 million (actual Jewish population)
then whatever Israel has in it's inventory, Tanks, Warplanes, Surface vessels, just times the total by 40%.
I hope this has been helpful.