I moved back to Australia from England when my daughter was about 17 months old (and I was about 23 weeks pregnant with my son) Make sure you get a cot for the flight – you`ll sit in the front row with a capsule like cot that attaches to the wall in front of you – they are a God send, the only negative about them is that baby needs to sit back on your lap when the seat belt light goes on – which can happen at any time, usually when bubs has just gone to sleep. Also I`d request to sit on one of the window aisles (not the middle rows) as there will be less chance of another baby sharing the wall – happened to us and the babies kept waking each other up. If breast feeding let baby suck when landing – helps with popping ears, otherwise give bubs a dummy to suck. I packed a little child`s suitcase (you know the bob the builder type ones on wheels?) with toys, books and snacks/treats. As for the time difference, if you have time start changing your routine a few days before you go – slowly staying up later and later – but being 9months old bubs will probably happily sleep? Obviously don`t forget bubs favourite blanket or toy.
Good luck with the flight and have a great time in Ireland!