Ferry and Rail

You can ferry to/from Dublin to/from Holly Head or Liverpool. The ferry to Holly Head is 3 hours. Liverpool appears to be significantly farther, I'm not sure exactly how far at this time, but is located near Chester, which I think was a potential destination. You can train from either Liverpool or Chester to London. It's like 4-5 hours.

Can get a rail pass for Ireland/England for $500 each, good for 5 travel days. Seems kind of pricey. I'd have to compare to individual train trips once we identify our itinerary.