In Holyhead (ferry port in Wales with many daily connections to Dublin) you board the ferry from the end of the train station platform of the high speed rail. The slow ferry takes three hours, the express ferry takes 1.5 hours. When I was there in 2006 there were many direct trains from there to London Euston.

The other option might be ferries that carry trains. I believe they don't do this across the Irish Sea, but I've been on a train that drives onto a ferry across the Baltic sea connecting Berlin and Malmo (Sweden). You can get off the train while on the ferry, but you don't have to.