Got a reprieve from the Summer Heat in Florida to play in Ireland in 2010, and got to play multiple courses. The bulk of the great courses are in SouthWestern Ireland and in Northern Ireland. I believe N.I. edges the rest of Ireland just because the concentration of great courses within a hour's drive from Belfast. SW Ireland is more spread out and will require more time to get to marquee course.
IMO, my top five for the Emerald Isle goes like this:
(1) Royal County Down. Just a reminder, the UK golf digest had them at the top spot, yes over St Andrew. The seaside holes combined with the Mourne Mountains backdrop are one two knockout punch.
(2) Royal Portrush (Dunluce). Only second fiddle because the natural urge to compare to its RCD neighbor to the south. The Antrim Coast and the course are a thing of beauty.
(3)Lahinch. A seaside links course with trickery around every dune. Once you play it more than once, MacKenzie's genius becomes evident.
(4)Ballybunion Old. Year in Year Out, always deemed the best links course in the Republic of Ireland.
(5) Port Stewart. Another gem in N.I. which is overshadowed daily by the giants that are RCD and Portrush.
Ahh, I'd like to get on a plane to Belfast for a long weekend and play RCD, Portrush, and Port Stewart again. I hear the shoulder season rates are accommodating, and airfare is way better than summer fares.