Looking at booking a September flight with AL.
My passport expires in May.
If I book now with a soon to be expired passport will that be a problem come September ?
in many cases you do as part of the customer details (esp Ryanair), but its usually just to save time when you are doing online check-in. But for validity to travel its the passport you have with you at the boarding gate that matters
Just in case the destination is the US, Aer Lingus will send you the request to complete the Advanced Passenger Information some time after booking and it can wait until you have the new passport in hand.