I'd have thought you should be due something for the downgrade, although if they reprice it against a full fare Y ticket then I guess you won't.
Not sure I'd have taken the switch to EI in Y though. Weren't there later LHR-DUB flights they could have put you on?
Perhaps your only hope is for the EI flight to arrive 10÷ mins late? Or does accepting the flight reset the clock somehow?