Ferries that accept foot passengers from Dublin go to Holyhead where there are regular train departures, except for the evening ferry services from Dublin c8-9 pm. which leave a long wait in a not very comfortable station with limited facilities.

https://www.stenaline.ie/

www.irishferries.com

Both offer some discount for interrail passholders on their standard fares but it may be useful to buy the local sail-rail offer that combines the ferry with rail to any GB destination and save a day’s pass use.

see the websites above for details.

Train times for Britain:

https://www,nationalrail.co.uk

It is much to early to book anything for May 2023 but looking at current times will give you a good idea of times and fares, they tend not to change that much.

Was not there also an overnite ferry Dub→ Liv? Or has this been cancelled? (Yes, I know I can google for it,  but then have to leave all this etc)

There are 2 companies running regular Dub - Liv sailings.

Seatruck takes mostly trailers without drivers and up to 12 truckers or car drivers/passengers per sailing. (under 12 paying passengers is still classed as freight ship, over 12 is passenger with stricter regulations)

P&O takes trucks, coaches, cars but no foot passengers.

You may be thinking of Belfast - Birkenhead (Liverpool) which takes foot passengers on 1 day and 1 night sailing daily, run by Stena Line. Combined sail-rail tickets used to be offered but were withdrawn some years ago. Interrail discount is offered on this route.