even if the Irish had entered the war, how meaningful would their contribution have been? could they have offered much in the way of military assistance to the UK anyway? it would likely have come down to the material contribution of bases, both naval and air, to combat the U-boats. so would the offer of the north have even been worth the trouble involved?
so imho the UK would have taken advantage of the situation and milked the agreement for all it was worth and then reneged on the deal at the 11th hour and kept the northern counties. and also likely, having had a good look at the irish armed forces, either decided to sell them arms or, if it was politically preferable, left them behind and not even offered anything.
if, however, they had acted as allies and had good relations at the end of the war then they would provide them limited numbers of modern weapons. even iotl they provided modern tanks long after the end of the war to ireland, just without spares or support.