Hi - we flew with Ryanair to Crete when DD was 4mo so hope I can help...
I don't know too much about Ryanair policies but we took a pushchair and a car seat. I think we paid a little bit extra to take the car seat (it worked out cheaper than hiring one there). We normally use a Baby Jogger CMGT but didn't want to risk getting it damaged so bought a Chicco Liteway for next to nothing (it's much lighter and has umbrella folding). The disadvantage of course was that we couldn't use the car seat with it. I guess it may be a good idea to make sure you are adequately covered for damage too, just in case.
While we were queueing to board, DD started screaming as she was hungry, which was a nightmare as it was almost impossible to sit down and feed her while we were shuffling forwards with all the other passengers. Make sure you allow plenty of time for feeding, changing etc - we failed here! I didn't find the airline staff particularly helpful (one cabin steward even told me he couldn't speak English and therefore couldn't help), and relied a lot on the good nature of fellow passengers to survive the journey.
When it came to boarding, I found it helped to be a bit assertive/sharp elbowed as there were no special arrangements for passengers with babies at all. We were lucky with seating both journeys though as got the very back seats on the plane - they have the advantage of a shelf behind the headrest and so lots of extra space to stow things you may need on the journey, plus were close to the loo (DD pooed constantly during the flight). Make sure you ask for a seat belt for the baby - we had to ask several times and thankfully one materialised just before take-off...
I second Squiffie re the sling. I actually took two - a stretchy sling which I had tied on me during the flight so that I could pop DD in there whenever I needed to, plus a Solarweave Connecta which I hired from a sling library. It folds up very small and was extremely useful when out and about during our holiday. We also had a folding sunshade for the pushchair, plus a large muslin and a couple of clothes pegs to ensure she was totally protected from the sun.
Finally, make sure you stay within the weight limits for your luggage - we had far too much stuff on our return journey and suffered the indignity of having to unpack everything at the check-in desk, watched over by a bemused DD sat in her car seat wondering what sort of incompetents she had for parents 
Have a good holiday - Ryanair isn't ideal, but you will survive!