The US takes less of a hit from morally dubious actions, and gain a lot more allies. The whole Red scare would be mitigated somewhat because America would be actively thwarting Communism with slow if visible results. As such, McCarthyism loses the steam it gained and less people are blacklisted...
I thought you'd rolled them into the United Kingdom which I would have accepted if there hadn't been home rule given to them earlier. I suppose a conservative government might revoke it so it works.
Tensions could erupt if Ireland isn't left as it's own dominion, and republican support could rise if the population feel they are being ignored. A country wide Troubles could happen, particularly if groups like the IRB or IRA still exist.