I don't recall anything as hilly as western PA anywhere I biked in Ireland. I think you'll be fine. I agree with Kayakado that the most picturesque areas are hilly and that they're worth it. These areas are generally in western Ireland. I thought the prettiest area was the Ring of Kerry area, including the Dingle peninsula and the Gap of Dunloe. It was touristy but lovely. The Burren was a strange and unusual landscape. As for rain, be prepared for it. Ireland had the worst weather I've encountered anywhere I've toured. There was some rain each and every day we were on our bikes. We were there in September, but locals said it rained all summer that year. It generally wasn't constant rain, though. I've never seen so many rainbows!