Immigration flying DUB to MAN

There is no routine immigration landing at UK airports on flights from the Republic of Ireland.

If I was you, I'd ask the airline where you go through Immigration. It's possible you go through it before you leave the US, in which case, no, you won't have to go through it again at Dublin.

My experience is by rail where I had no customs/immigration checks between Ireland and Northern Ireland.

My experience is 10 or so years old, we flew Leeds to DUB and back, and even with the CTA, we needed to go through Immigration both arriving in Ireland and again returning to the UK. Residents of the UK may not have had to, but non-residents traveling by plane usually do.

Read what Marco said. Memorize it. Repeat it to yourself. Carriers vary on what you need. Ryan is the only one that requires a passport. Stems is happy with a water bill. Everybody else is in between. That's just to get on the whatever. There is no immigration control. Flights from Ireland come into Manchester through the domestic gates. There ain't no border fuzz there. You can drive from the Republic into Northern Ireland and you have to squint to see the border sign. You can tell you made it by the speed limit sign switching to miles and the density of speed cameras increasing. Read what ....... When you get into the main part of the terminal look for the sign that sez STATION. If the interior passage is clobbered with people and you're nervous about time, go walk outside where it's less crowded. Sheesh, doesn't anybody sleep in airports and snoop around while they're waiting for things to quiet down?

The UK views the CTA differently from the Republic of Ireland. One thing worth adding though is that if anyone needs to have a visa activated or registered on arrival, such as a student, they must arrive directly into the UK and not via Ireland, as the Irish authorities can't activate a UK visa.