There were scaremongering propaganda maps saying Austria-Hungary wanted to turn it into a Hapsburg colony (presumably because Catholics) but there wasn't any truth to that. I have always taken the position that (ironically) a CP victory would almost certainly leave all of Ireland under the British crown, because:
1) It's not an area the CPs would particularly care about (if they are in a position to hurt Britain, they would rather strip her of prestigious colonies in Africa or Asia);
2) While they can (and did) smuggle weapons to Irish rebels, the Germans don't really have the power projection capability to enforce an independent Ireland;
3) Furthermore, the CPs would gain little except maybe naval bases from an Ireland on their side, but a restive Ireland reconquered by Britain would allow them to keep funnelling aid to rebels and turn Ireland into an ulcer that would weaken Britain for years to come and help keep them out of European affairs;
4) The rebels in Ireland had taken on a strongly republican character years before and would not accept a king even if it was the alternative to British rule;
5) Britain would be humiliated after defeat in WW1 and would want to snatch an easy victory in her own backyard to save face (and as that Irish guy on Cracked.com hilariously commented in his St Patrick's Day article the other day, Ireland has always been the tutorial level in Britain's game of Civilization IV, Conquest Victory mode).