Originally Posted by
ISTFlyer
In this case, purchasing a multi-city ticket might be cheaper rather than two one-ways since the OP is flying DUB-LCY/LHR-MAN within 24 hours, the APD for LHR-MAN wouldn't be due as he is originating from the Republic of Ireland for that portion of the trip.
I think for INT-DOM connections it is 6 hours, although if arriving from the international between 1700-2400 you get until 1000 the following day.