John:
In the end, it's all Ireland and it's all good.
But having made a few trips over, what I would advise is this:
NI has much better roads - so if you are planning to get around a bit in your short 4-5 days, that might help.
From the US, you'll have far more direct flights into Shannon or Dublin than you will into Belfast, so that could save some time at the beginning... and several great courses are right near Shannon, allowing for day of arrival play, which I consider mandatory.
For drinking purposes, you're not gonna find a bad place really. That being said, as Paris is to fine food, wine and art, Dublin is to pubs. A night or two in Dublin is an experience not to be missed.
But that being said, one can find fun wherever one goes and the best just might occur at the most out of the way places....
You can see where I'm going re this. Pick the courses you want to play, see which work out best, go from there. Any way you do it you can't really go wrong.
Oh yes - you need to plan WAY in advance if you want to play Ballybunion and some of the other big names any time in the summer. NOW might even be too late for next summer - you're going to have to be flexible. So if you do ROI, contact Ballybunion TODAY and then work everything else around that.
TH