Thanks Tonty - I see your point. Yes, the plan is to do LHR - DUB on day one, overnight in DUB and then DUB - LHR - PHX on day 2.
Was also thinking abut luggage, and have just had a quick look at BA flights for the LHR - DUB leg. Economy flights are something like £75 ( maybe 74, but around that figure), but are carry-on bags only. They want to charge £65 for a checked bag - however, they also sell economy plus flights, which are about £90 or maybe slightly more, but that does include one checked bag, up to 23g. Much better option, methinks - we may well come home with more weight than that (both bags and personally!), but going out will be fine. Unless I've missed something glaringly obvious.....