Speaking from my own experience (mostly to/from France and Spain, sometimes direct, sometimes via Stansted...) - I don't think they'll stop you from bringing a normal pram/buggy - or even two of them - as far as the 'plane (sensibly enough, because depending on the airport it can be a 1500/2000m walk from the check-in area to the Ryanair gate...). Then you leave it/them folded up at the bottom of the boarding steps, and recover it/them likewise at the far side. If your car seat attachment isn't needed, they'll probably make you check it in through the oversize/delicate section - in which case, leave yourself plenty of time after check-in!
But - if you're travelling with two small kids on Ryanair, please, please spare yourself a lot of grief and (at least)
double the amount of time you think it'll take you to navigate through check-in/transfer procedures. They board passengers with young kids first, but when you get off at the far side (and particularly if you've a 1500/2000m walk to baggage reclaim, and then have to check in all over again for a 'connecting' flight (Ryanair don't 'do' connections...!) - there's nothing worse than watching that new check-in deadline roll around while infant X suddenly decides to have a vomiting/screaming fit, or needs an urgent nappy change. Believe me, I've been there!
Oh - and stock up in advance on anything you might want/need during the flight (water/bottles/soothers/food/toys/colouring books, etc.). Don't rely on the inflight 'service' for anything... not even to warm a bottle!
[P.S. When boarding, try to get seats near the front or the back of the plane - makes for a slightly quicker exit at the far end...]