I took this trip with @geekyjames a while back, we also visited Holyhead Lifeboat while we were there. I've always wanted to go to Holyhead, not sure why - I think it's just one of those corners of the UK that needs to be ticked off. The station at Holyhead is directly adjacent to the Ferry terminal and (unsurprisingly) almost all of the passengers headed this way when we arrived.
The Wales North Coast Line is jointly operated by Arriva Trains Wales and Virgin Trains. Arriva run stopping services towards Cardiff Central, Birmingham, Maesteg, Shrewsbury. Virgin run intercity services via Chester towards London Euston.
The line isn't electrified so Virgin can only run the 221 Super Voyagers to Holyhead,however they have been known to use the Pendolino sets led by a diesel unit.