Well LM, how's she cuttin?
First off I wouldn't recomend walking on Irish roads it's not safe. They twist and turn and go up and down and in most cases your walking on the actual road. Short walks are fine and many towns have walks you can go on which are more than likely littered with ruins and ancient sites. Inbetween places take the bus, there's loads.
I hope you know your stuff though allot of the ancient sites look like bumps in a field. My town Dunmore (Dun Mor) means large fort and you can still see the original fort altough it's, like I said a hollow hill in a field. You'd never know from looking at it that it was part of a settlement.
Places of interest are the hill of tara,
Waterford has rebuilt one of the ships that took the Irish to America during the famine,
Negrange is probably the best ancient site in the country.
More ancient sites
There are town's that still have much of the original walls left from medievil times and I can't for the life of me remember them...
Try google.ie for more information they have an Ireland only button. The government has a tourisim site but i don't have time to find it at the moment.
Oh! If your comming to Galway, see the aran Island there's an amazing fort on it.