Connecting United to Aer Lingus Flights at LHR: Can I make it?

Yes this connection is perfectly doable on a single booking. Follow signs for connections rather than arrivals. All in the same terminal helps a lot, but not that you will need to go through security as well, which will involve also taking your photograph to be matched at the gate.

In the even of you missing it, most probably because of a late incoming flight, as you are on one ticket you will be re-accommodated on the next available flight. This applies with your SFO connection as well.

Please check the Aer Lingus website for regulations on cabin luggage. The restrictions on this short, internal British hop may not be the same as United trans-Atlantic. Carry-on bags may also have to go through a security inspection at a central point rather than the boarding gate; these group security points are often crowded.
Heathrow is large, and recent events have made officials security-conscious, slowing down movement. It may be small comfort, but you can check the schedule for Aer Lingus to Belfast to see when the next plane will fly if you miss the connection. Print out the schedule; it won't hurt to also print out appropriate terminal maps for Heathrow (or maybe download?)

Next time, take Icelandic Airlines from Portland to Iceland non-stop and a direct flight info Belfast. It is seasonal, but maybe when you're going. Then you won't have to deal with LHR.

Try not to be nervous. You were smart to book this on one ticket. If something goes wrong, it is up to the airline to get you to your final destination if one of their flights is late.

Since you've already been to the Heathrow "flight connections" website just follow the instructions given. If you need help. there will be people who can help. Just ask.

This connection is definitley doable. Since you can't control the situation, worrying is not going to help. Just arm yourself with the knowledge of what you need to do and think about the great time you're going to have.

Frank II could not be more correct, on all these trips be prepared for the fact that things don’t always go according to plan, just make yourself relax and realize “I going to have a great time in Ireland!”.