It is a messy business adding an infant to an existing booking. Cannot be done on the computer.
You must -with EI - contact the airline, and ,if memory serves me right, it's even more difficult with Ryanair.
I'd urge you to read the rules on the two websites now -or else choose a name now.
I can't recall Ryanair slashing high season fares for the more popular destinations a few weeks beforehand.