I'm not saying Ryanair does or should have interlining agreements at Dublin, however to make up the lost TATL passengers, Ryanair would have to come to some sort of agreement with Aer Lingus to help facilitate and regain the lost TATL pax.
Many PAX on Aer Lingus tend to be TATL, if not, BA codeshares/avios points and business passengers. Leisure passengers seem to be more towards Ryanair.
Interlining agreements at DUB could also prove popular with other UK regional airports where Ryanair has early flights to Dublin such as BHX, EDI etc.
So now we've been left with LHR, CDG, AMS, CPH, BRU, DUS where we have airlines (in Europe) that facilitate interlining/transfer.
With Aer Lingus dropping TATL from NCL and not so great timings, I'm not holding my breath. This one flight a day with EI seems to carry on right through to May 2018.
NE