Fantasy novel with American man visiting Ireland & going to Hell

I read this book as a teenager in the 90s, I think it was written in the 70s. The main character is an American man who goes on a trip to Ireland to see where his family came from, and ends up interacting with various Irish myths and going to the Christian Hell. I believe it ends with him interacting with the Blarney Stone in a way that sets up a possible sequel, but I have no idea if there were any.