Ferry or plane?

Getting from London to Dublin involves a train (leaving from Euston station) to the coast and then a ferry to Dublin. Total travel time is about 8-1/2 hours not including getting to the London station and to Dublin from the ferry port. Looks like the better part of a day. Flight would definitely be shorter even with wait times at the airport although it probably would be more expensive. I'd research the options carefully before making a decision.

Fly.

I'd start in London, then take a train to Edinburgh (or whatever Scottish destination you prefer), then fly from Glasgow or Edinburgh to Dublin, and fly home from Dublin. Personally, I'd avoid ferries. They're far more time-consuming than airports. Flights between Scotland and Ireland are frequent and cheap.

Fly. Look at the lo-cost inter EU airlines that you can find on Skyscanner.com, being careful to review luggage limits/weights. Don't forgot to look at flying into one airport and out of another; i.e. into Dublin , and out of Shannon, etc.

First, London is not a country. Second, while the advice above to fly is probably correct, in order to really answer your question you need to decide where else you're going to be spending time - are you really only going to be in London (the city), or do you plan to visit other parts of England? If, by chance, you were going to be near the ferry port anyway (because you were planning to drive there because you wanted to visit that part of the UK), then it might make sense to take the ferry. The train may or may not be best for "Scotland" - if you really just want to just hop from London to Edinburgh (without stopping to see anything in between) then you may want to fly there, too. I think there may be a larger, unspoken question in your post: how much of that "valuable time" (that your buddy doesn't want to waste at an airport) do you have? How many FULL days do you really have there, not counting your arrival or departure days? Start with that hard fact, then figure out where you want to go (not "countries", you need to be more specific) - and what's realistic. Only then can you determine what mode(s) of transportation make sense to string those places together.

Another consideration - how good a sailor are you? The Irish Sea is notoriously rough at certain times of the year.....