American Phone

Hi, is it possible to tell if an UNLOCKED american phone will work in Ireland. It is a Galaxy S6.

It depends. Is it set up for Cellular technology support on either CDMA (US) or GSM (Euro & other) networks or both?

There's no technical reason why the Galaxy S6 wouldn't work in Ireland since the advent UMTS I think. (W-CDMA is that standard to be precise)
Smartphones radio chips usually support all frequencies that might be in use worldwide for W-CDMA.
I seem to remember the CDMA / GSM issue that has plagued us previously was only 2G and 2.5G, and has gone away with 3G networks pretty much being available everywhere now.

So Smartphones (especially news ones like the S6) will pretty much work everywhere.

One problem that I came across recently is that if it's within warranty and there is a problem with it, Samsung Europe will not deal with the return/repair. I had an S6 purchased in Africa and it had to go back there for the repair.