Funny you brought this up, as we have just been thinking about organizing a trip to Ireland as well. We like to visit ancient sites. The first thing I always check when planning a trip is the UNESCO world heritage list:

http://whc.unesco.org/en/list

There are two historical sites listed in Ireland: Skellig Michael (in Kerry) and Brú na Bóinne (just north of Dublin). The ancient fort of Dún Aengus on Inishmore (the largest Aran island) is supposed to be really nice as well.

Besides that, pubs can be found everywhere Though I would definitely visit the Guiness brewery on any trip to Dublin. It'd be like drinking from the font of all wisdom...

BTW, there is also a ferry between Ireland and Scotland...runs from Larne in Northern Ireland to Troon, and it's not expensive. Link here:

http://www.poirishsea.com/passenger/Larne_Troon.htm

I, for one, would be hard pressed to avoid making my Irish holiday into and Irish AND Scottish holiday!

Cheers