Traveling from Dublin to London as a US Citizen

There is no immigration or customs from Dublin to London.

There is no arrivals immigration or customs if connecting in Dublin.

However, you will be doing US preclearance in Dublin. That means you will go through U.S immigration and customs in Dublin. When you get off the plane in America, you just walk off like a domestic flight.

This guide will help you:

https://www.dublinairport.com/flight-information/connections/connection-guide

Last summer I flew Seattle to Dublin via Heathrow. I missed the direct shot to the Heathrow to Dublin part and had to go through security at Heathrow. It worked out okay because the plane was delayed and I didn’t have to do anything but walk off the plane in Dublin. I don’t know if that would've been different if I hadn't screwed up.

Fortunately, instead of choosing the fastest connection I made sure I had plenty of time between the flights. I carried on a 2-wheeled bag and a small backpack. I checked a little duffel bag primarily for my hiking poles. It made it through to Dublin just fine.

I took the ferry from Belfast to Liverpool and was surprised when I arrived that we had to show our passports before proceeding through the little terminal building, even though we were traveling between two UK ports and passports were checked before getting on the ferry.

My advice is to allow as much time as you possibly can. Typically you'd need to be at the Dublin airport at least 3 hours before departure for the US. To me that means allowing plenty of time for potential delays of the flight from Heathrow to Dublin. If both flights are on the same ticket, you'd probably be covered if a 1st flight delay caused you to miss your flight home, BUT it could be a big hassle if you have to take a different Dublin to US flight.

If they aren't and you're doing a separate LHR--DUB after your time in London, it gets a little dicier. If your flight back to the US is early in the morning I'd be inclined to fly back to Dublin the day before, spend the night in a hotel near the airport and avoid the worry about the connection.

BTW, last summer I actually did a "multi-city" itinerary, SEA--LHR--DUB and LHR--SEA. If you already have RT US--DUB--US tickets at a super price, I can understand why you'd want to go back to Dublin to return home. If not, you might consider multi-city, US--DUB, DUB--LHR, LHR--DUB, DUB--US all on one ticket or US--DUB, DUB--LHR, LHR--US all on one ticket.

Whatever you decide, have a great time in Ireland and the UK.

It is routine for passport checks on the Belfast to Liverpool ferry route, although normally at the Liverpool end, rather than Belfast. These are police checks, not Border Force checks, and have doubtful legality.

Strangely those checks are rarer on the Belfast to Cairnryan ferry.

Note that the Cairnryan ferry and the Liverpool ferry have different terminals at Belfast, known as the VT5 and VT2 terminals respectively (VT for Victoria Terminal)

From your other posts it sounds like you will be self-connecting on your return to the U.S. In other words, you have roundtrip tickets US-Dublin-US. Then you will separately travel from Belfast/Dublin to London then back to Dublin. If that is the case and you are checking luggage make sure you give yourself plenty of time in Dublin to catch your flight back to the U.S. You will need to follow the self-connecting portion of the guide Frank II linked to. If you are self-connecting you will have to go through Irish immigration, claim any bags, re-check any bags, go through Irish security and then US TSA security and US immigration. You will need at least 3 hours.

There is no immigration or customs from Dublin to London.

There should in theory be customs from Dublin to London, although it seems like the UK customs are more or less ignoring goods and passengers arriving from the EU.

There is no customs or immigration from Dublin to London normally, because the Irish Republic and the UK are in the CTA- the Common Travel Area - (which is the UK, the Irish Republic, the Isle of Man and the Channel Islands).
But there can still be identity checks.