Like bhxboy, I flew an outbound with FR and then inbound with EI, a little while ago.

Ryanair had a clear vision of what level of service they wanted to provide – it was budget travel, plain and simple.
Aer Lingus didn’t seem sure what they wanted to be – low cost or full service.

The most noticeable difference was the quality of the inflight service that EI provided – it was genuinely warm and represented the nationality of the country – as you might expect from a flag carrier. FR were decidely unimpressive.

On the ground, the service from the EI staff was patchy – some good service from someone whose job was to help out with the check-in process, but the service by the airside groundstaff of EI did not match up to what I had experienced with FR (who had contracted out their services)

As for food prices… not much between the two, if I remember correctly, Aer Lingus were a little cheaper.

FR operated more punctually.

I found the comfort levels on FR better than EI (seat comfort similar, and leg room as good or better)

Overall there was little to choose between the two.

Taking a balanced overview of the whole experience, then EI were the better operation – the single factor winning it for EI over FR being the excellent inflight service.