Traveling from US to Dublin in April.

You will follow the purple signs that say “flight connections” after you depart the plane. Eventually you arrive at a location where there are two different lines. One for international connections and one for UK/Ireland connections. You will show your passport and boarding pass to an agent, you will be sent through security again, and then be in the airside part of the terminal ready to go to your gate.
https://www.britishairways.com/en-us/information/airport-information/flight-connections

I think your connection time is doable, but you want to stay focused and keep moving as quickly as you can. Make sure you get rid of any water or liquids you picked up on the plane before you go through security again.

What Carol says applies if on a single ticket.

If 2 tickets, with checked bags, you could be in deep doo-doo

There are 3 of us traveling, each with one checked bag.

If you are on one ticket, then your bags will be checked through to Dublin and it won't matter that you have checked bags. If you have two tickets, you would be adding another lengthy step to the process. That is why people are asking that question.

Two tickets means that you purchased your ticket in two separate transactions, one for each airline. One ticket means that you purchased your flights from one airline and are simply routed on the two different airlines by that airline.

You will also have to change terminals. Some of AA flights arrive at Terminal 3 and some at Terminal 5. Aer Lingus is in Terminal 2. And since you are no longer airside, you can't take the airside bus.

You go through immigration at Heathrow and again in Dubin. (British friends here will jump in to say there is no immigration between England and Ireland. That is true, for British and Irish nationals only. Americans have to go through immigration. I have done this many times.)

Use this site to get step by step instructions at Heathrow:

https://www.heathrow.com/connecting-flights

Thanks so much for asking this question, robbbg. I haven't made flight reservations yet and I always do carry-on, but the terminal transfer issues are the same.

I arrive at Heathrow from Seattle at 12:30 and I've been tilting toward scheduling the Aer Lingus flight to Dublin that leaves 3+ hours after that instead of the 1+ hour squeaker. It will be on the same ticket.

The responders to your question, along with the links, made that decision much easier for me. I learned some logistical info that I had no clue about. A classic case of "you don't know what you don’t know." 😉

Thanks so much everyone. What I realize now is we may not have had planned well enough on that connecting flight from Heathrow to Dublin!

Really appreciate the information.

The connection is do-able, and I’d hustle to make it. But between Aer Lingus and BA there are SO many flights a day, if you do miss it, you won’t have to wait long for the next flight options.