from dublin to wales to london

Trains go from Holyhead to London (Euston) via Chester. Read this:>https://www.seat61.com/Ireland.htm

You would have to hire a car at Holyhead to see Wales and then onward to London. 1 day is not really enough to see even just north west Wales. You would just have time to drive across Anglesey on the A55 expressway and for scenery, I would take the A5 via the Nant Ffrancon Pass*. You could continue on the A5 to Betws-Y-Coed - which would be good for an overnight. Next day continue on A5 to Llangollen and then over the border into England for the drive to London - but you could perhaps drop off the car at Shrewsbury - have a look around the town before taking a train to London.

• You could take a longer drive by going first to Caernarfon (castle) and then A4085 to Beddgelert > A498 & A4086 to Betws-Y-Coed. http://www.caernarfon-castle.co.uk

If you don’t want to hire a car, take a train to Bangor, bus to Caernarfon and go on the Welsh Highland Railway to Porthmadog. Stay overnight in Porthmadog and then a ‘normal’ train to London. This line is scenic and slow as it goes down the Welsh coast before turning inland and heading east to Shrewsbury and on to Birmingham. www.festrail.co.uk

www.nationalrail.co.uk
To find car hire companies at Holyhead try:>https://www.travelsupermarket.com

If I said I'd like to enjoy Ontario for a day what would your response be? I doubt you'd even advocate devoting a day to Toronto as anywhere near sufficient. You simply can't visit Wales in a day, sure you can drive through it but that's not really visiting, you may as well just fly from Dublin to London and given that you only have a day why bother with the expense of a hire car, ferry and accommodation for what is essentially a stopover?

thank you for the response . I will rethink on it

I might play devil's advocate to the others.
Depending on what car rental you use (my husband and usually use Enterprise). Perhaps you could rent the car in Holyhead and drive to Chester via Conwy. There is a marvelous castle there the children might enjoy. Then drive to Chester, walk the city walls, visit the grab a meal, turn the car in (not necessarily in that order) and take the train to London. The train trip is about two hours and can be done with no changes.

You don't say when you are going but buying the rail tickets three months in advance is less expensive than day of tickets.

James you can jump in because you know more than I....If I were to buy tickets Chester to London for tomorrw for 2 adults and 2 children it would be 248.70 pounds. VS in three months about 100 pounds.

Just throwing this out.

Have a good trip.

I like Ann's idea and I think it's doable if you weren't planning to be in London until late in the day on the 4th. BTW Irish Ferries have cabins at a reasonable price with actual beds (ours had 2 twin beds and a big couch) where you can get a good long nap. We arrived in Dublin at 05:30 and were happy to get a few more hours of sleep mid-morning on the ferry. It made a big difference in the quality of our afternoon after a night on the plane.