Passport Control in UK, Ireland

Just the opposite.

You will go through immigration in Dublin but will not have to go through immigration in Manchester. Entering Ireland also gives you permission to enter the UK.

The flight from Dublin to Manchester is just like a domestic flight.

Frank, thank you.

not knowing how long your stay will be, by entering via Ireland you will have the Irish admission length which is lower than the UK 6 months.

You will still need an ETA.

Nigel is correct. If you entered the UK directly, you would get up to six months to stay without a visa.

However, entering via Ireland you only get 3 months.

If you are returning the same way, you will go through Irish immigration in Dublin, and the go through US immigration in Dublin as well. This way, when you get back to the US, you just walk off the plane. No customs or immigration.

it should be noted that CTA rights only apply to UK (also include nearby Crown Dependencies) and Irish citizens, for everyone else it's just slightly less hassle when travelling, and that's doubtful in some cases! One occasion at Manchester some time ago, in order to avoid going through passport control after having walked off the plane via an airbridge we were disembarked via steps, put on a bus, driven around the building and then had to climb more stairs back to the baggage hall! Given that everyone actually needed something as good as a passport for the security checks before boarding the plane the only hassle it really saved was for the immigration staff who didn't need to set up an Ireland arrivals lane!!!

CTA arrivals are not just like domestic flights because customs rules STILL apply (in fact they even apply for travel from the Channel Islands to UK), but normally few people get checked in either direction.

Basically the immigration controls work on an honesty basis, you need whatever you would need if you arrived directly in the UK - and if you haven't got it you are breaking the law. So ETA and visa rules apply, people banned from the UK are still banned (one american right wing activist who was recentlly banned has announced she will still come - that's going to end in a stay in a cell if she does). And flying back to Dublin they are stricter on "identity checks" when you arrive, which isn't the same as immigration checks, is it?