Flying with a newborn - UK moms

Shannon
Does anyone know the earliest you can take a baby on a plane? 
I have a 5 week old who isn't registered yet due to the office having late appointments but as soon as he is, I want to get him a passport.
I live in the UK and I only want to go over to Dublin which is an hour flight - if that. Has anyone had experience of flying with or without a passport??