I'm with tonyb on this one, I think you should allow a frozen turkey at least 24hrs to defrost before cooking it, also if you are arriving into Ireland at 17:30hrs by the time you clear passport control / baggage reclaim / customs and then collect the car, you are leaving it a wee bit too tight to locate a supermarket. If you do cook it from frozen on a medium heat it should be ready to eat around new years eve.
Good point about opening times, littlefox. If my memory serves me correctly, most main stores closed at around 6.00 p.m. (18.00) on Christmas Eve in Dublin when I was last there at that time of the year. Smaller, family-run shops closed even earlier. So yes, OP, your assumption in #3 is probably correct.

Tesco would probably be your best bet as the 24 hour ones close around 6 or sometimes 8. check out tesco.ie. I dont fancy your chances of getting a frozen chicken tho.
best of luck
happy christmas