I will be escorting my wife and my mother-in-law (her first trip outside of the USA) to Dublin (3 days) and London (5 days) but in between we have a full day after the ferry from Dublin to Holyhead. I personally would like to see York Minster, but I can see the attraction for the Beatles history for many. The trip is tentatively scheduled for first half of September, so, no rush, but I'd like to get some ideas or possibly alternative one day interludes for our free day between Dublin and London.
We had also considered Chester, Llandudno, and Bath.
Thank you for the posting, Emma. We are flying into Dublin, then the ferry and the train, we are big followers of Great British Railway Journeys and the George Bradshaw guide with Michael Portillo. I had been considering a full day after arrival on the ferry/train, so, I think we will have a full evening and the entire next day before continuing on to London.
If you were going by car instead of train and caught the early ferry to Holyhead, York might be doable since driving is considerably quicker than the train. As it is, York is quite a stretch compounded by the weather. If the weather is bad, the high speed ferries don't run and you will be making a four hour crossing. We spent a day in Conwy which is near Llandudno and while we drove over to Llandudno to do our laundry we found Conwy which is w walled town to be much more interesting. Catch York next time, it is a really nice town with a lot to see and it is easy to do it all on foot.
I would not go to York - which is quite a way off your route and the journey passes congested areas.
My choice of break would be Conwy - which has direct trains from Holyhead but inform the conductor that you wish to stop. If on a Virgin express train, it will not stop at Conwy but at Llandudno Junction which is about 5 minutes further and the place where you change for the seaside resort of Llandudno - which is also recommended for a stay over. If staying in Conwy or Llandudno, you could actually take one of Portillo's rail journeys - to Blaenau Ffestiniog and then transfer to the narrow gauge Ffestiniog steam railway. (If you got off at Minffodd, you could walk the 1 mile to Portmeirion but make sure you can get back to the train). Alternatively, you could stay on the train to Portmadog and then take The Welsh Highland steam railway to Caernarvon - see the Castle, then bus to Bangor and train back to Conwy/ Llandudno. If you don't know where these places are - use Google Earth.
www.festrail.co.uk
http://www.portmeirion-village.com
www.nationalrail.co.uk
www.arrivatrainswales.co.uk
www.greatorme.co.uk
http://www.visitllandudno.org.uk
www.visitwales.co.uk
www.visitengland.co.uk
www.castleexplorer.co.uk
http://www.nationaltrust.org.uk/bodnant-garden/
Chester is well worth a look. You could stay and travel from here to Liverpool. http://www.visitchester.com
You're getting good advice here, I think. York is a stretch, save it for another time. Chester is a wonderful city, interesting cathedral, walk around the old walls, lots of timber-frame buildings, good museum. Conwy has great castle ruins and walls, but might complicate your trip too much to stop there and Chester on the way to London.