Have opportunity for good fare..United to Chicago and then Aer Lingus ORD to Dublin. Not familiar with Aer Lingus, which does not fly from my home city.
Can a person make seat selections at time of booking? If there is an extra charge, is it very expensive? I must have an aisle seat due to leg injury and refuse to fly on any airline that won't allow advance seat selection.

Advance seat selection is not by airline - it is often by flight and by date - when a certain number of seats have been reserved most airlines stop seat selection. However, if you have a specific issue yuo might be able to reserve a seat. You should call them and ask.

I flew Aer Lingus last year and was able to select seats in advance on the web site. This year I made reservations thru UA but the flight is on Aer Lingus and selected seats, but thru the UA website. The Aer Lingus website should give u information about seat selection and of course, calling would work 2.

Yes, when we booked our AerLingus seats I used their seat selection. We like to be near the front of the aircraft.
I think you can also go to seatguru.com and that advises about bad seats as well.

I don't know about from the US, but from Europe Aer Lingus charge you for selecting seats, not a lot but they still charge.
A lot of European airlines are now doing this for trans-Atlantic flights too so it you should certainly check their website, and if they do charge decide if it is worth it.

Aerlingus.com charge small subcription to pre- book seat allocation for flight, if you do dont pre book your seat then seat number is allocated at check-in free of charge.