I love Ryanair!
Points in Ryanair's favour:
1. They're cheap.
2. They've got the best reliability record (and the lowest number of lost bags) of any airline in Europe.
3. They've got an excellent safety record (as, indeed, has Aer Lingus).
4. If you're flying with only hand luggage (as over 90% of travellers on their UK-Ireland routes do) you needn't worry about their baggage charges that some people like to moan about.
Points against Ryanair:
1. You can't check in at the airport (but why the hell anyone would want to is beyond me anyway - doing it online is so much simpler). Actually, you
can now check in at the airport but you'll get charged £70/€70 for doing so!
2. You can only check in online free of charge up to 7 days before a flight, which can cause difficulties for people going to places where they'll have no internet access (complete with a printer) to check in for their return flight. Ryanair's online check-in facility is available up to 30 days before a flight only for people who are prepared to pay £5 (or €5), per person per flight at low season rates, to reserve their seats. (Aer Lingus offers free online check-in, to all passengers, up to 30 days before a flight).
3. Ryanair's baggage fees can sometimes be higher. Scroll down here to see their fees:
http://www.ryanair.com/en/fees/
and see here for the charges levied by Aer Lingus:
http://www.aerlingus.com/travelinformation/baggageinformation/checkedbaggage/#checkedbaggagefeestable