If arriving by UA, just proceed to flight connections, do security, up the stairs, be sure to go with the UK+IE lane so you get the biometrics done and they will put a sticker on your passport with a number.

Then find the EI transfer desk on the right as you exit from the passport area

If no one at the Transfer desk, proceed to gate A21-23 and find a gate agent, or if you have status with EI or an Aer Space class ticket, the EI lounge staff will issue a boarding pass. But as above checking in via the app makes life a lot simpler.