London to Dublin

Is this worth doing ?

Only you can answer that. If you have the time and money to avoid a dull 75 minute flight, have at it.

Stops on the way ?

Depends on how much time you have.

In my opinion tne best way is to fly. There is no "fun" way of getting there.

Of course if you have several days available to make the journey then by all means take the slow option and see more of the UK en route rather than spending those extra days in Dublin.

Fly directly to Dublin and spend those extra days before the tour in the area around Dublin not included with the tour…..just my opinion.

There has been no Liverpool to Dublin passenger ferry service for around 20 years, although there is still a Liverpool (well Birkenhead, across the river) to Belfast service.
Ferries leave from Pembroke and Fishguard (in South Wales), Holyhead (North Wales), and Cairnryan (Scotland- sailing to Belfast).

Most of these ports don't really have car rental facilities close by- anyone driving over uses their own car, and taking a rental car over will cost a fortune due to one way international drop off charges. However, if you could swing it, the old way to Holyhead is still by far the best- use the old A5 rather than the M1/M6/A55 route. The A5 parallels the M1 for a long way out of London, and is a routine diversionary route when the M1 is closed. But it does pass through a lot of interesting places in the Midlands. In Wales it cuts through the heart of Snowdonia so is a pretty scenic route. You could do it in a day, but is really at least a 2 to 3 day road trip to do it justice.

However one of the best known secrets of the UK rail network is sail-rail tickets. Rail and ferry travel on one ticket-
London to Dublin Ferryport £53.40
London to Rosslare Ferryport via Fishguard £62.40 (about 8am from London Paddington)
London to Belfast via Glasgow and Cairnryan £63 (this route only also gets railcard discounts so under £45 with a railcard)
(even that route can be done in the day with a very early start from London)

But if you use normal train tickets there are any number of places you could break the Holyhead route in North Wales.
On the via Pembroke route you could on Day 1 go to Cardiff or Tenby (or elsewhere in South Wales), then on to Pembroke on Day 2 (there is also a night sailing from Pembroke)
Likewise on the via Fishguard route on Day 1 go to anywhere in South Wales (but St David's would be a very good idea), then bus from St David's to Fishguard for the ferry. This route also has a night ferry, which connects into a train for Dublin (extra cost).
On either of the above day sailings you would probably want to overnight a few miles away at Wexford, then train to Dublin next day.

If you go via Scotland there is a same day rail connection from Belfast to Dublin. But equally lots of places on the way to Cairnryan for an overnight or longer break of journey.

But if you are looking for a fun way to get there and have a few days to make the journey you could fly from London to either Campbeltown or Islay (both Scotland) connecting at Glasgow. A day or two for distilleries and other sightseeing on Islay or at Campbeltown.

Then the Kintyre Express ferry from Islay to Ballycastle (Northern Ireland)- a 1 hour journey, or Campbeltown to Ballycastle (90 minute journey).
Ballycastle for Rathlin Island and the Antrim Coast. Bus to Portrush or Coleraine, then train changing at Belfast to Dublin.