If you are interested in ancient ruins, make sure to visit Newgrange. It's a restored passage grave that is over 5,200 years old. That makes it older than the Pyramids or Stone Henge.
If you can make it out west, try to visit the Aran Islands on the Atlantic coast. You will love Dun Aengus (The Fort of Aengus), an ancient stone-age fort perched on the side of tall cliffs, and surrounded by thousands of granite spikes as a defence works.
Those are the two sites that jump to mind. You'll love Ireland. Spend at least one weekend in Dublin. If you're looking for your ancestors, drop me a line. My father runs an Irish genealogy society. It's quite difficult sometimes though, as most of the Irish church records were destroyed during the Civil War in the 1920's. I'd be more than happy to put you in touch with the Irish Society of Genealogy though...
Mr Mephisto