Flights with booking.com - price seems too good to be true

Yes used them multiple times to book flight and hotel packages. What I find bad about them is usually the base price for the flights doesn't include hold luggage for long haul flights so you may have to pay for this seperately depending on the airline, have had occassions where half the flights had no extra charge and the others did at €50 or so a bag.

The difference in yours seems out of sync, is there a chance it's a codeshare flight and that another airline are actually operating it and so their price is cheaper for the same flight than Aer Lingus are charging?

I've had it happen a couple of times, once with Aer Lingus and once Lufthansa, that a rather expensive European flight was an amount cheaper through a 3rd party, but more like 400 down to 250 or 300 so still damn expensive. As said above, it was down to codeshare or some other quirk that you can get flights off the shared Amadeus system for an expensive price rather than very expensive direct from Aer Lingus or Lufthansa

On top of that, when looking at flights on skyscanner, often 3rd party services are quoting prices from a cache from a search days or weeks before, so that €100 offer may turn out to be a false alarm when you get as far as the confirm flight stage.

The main problem with OTA's is that they do not have live pricing so what seems like a great price is not so great when they come back and say 'sorry this fare has increased'.

While Booking.com is well established, they only started booking flights 3/4 years ago and at that time were using GoToGate, a notoriously shonky agent that I wouldn't touch with someone else's bargepole.