OP Here - OK, I stand down from my initial disbelief (but not my annoyance). It's a stupid rule, I must say. BA has an arrivals lounge in London but also grants access airside to passengers in transit if you arrived in business on a longhaul flight. I really did need a shower! DAA actually advised that I could use their T2 lounge which has shower facilities, but by then I had trekked back and forth several times and was more in need of caffeine than a shower by that point.

I also note that had I known a bit better, I would actually have entered through normal immigration and used the arrivals lounge - the transfer immigration from the transatlantic to the local connection to London only had one person checking non EU passports for flights to UK and Ireland - it was about a 45 minute wait to clear. I can't help but feel that going through normal immigration, using the arrivals lounge and then going back through would have been at least equally time consuming, and there would have been a shower waiting to boot. The DAA lounge was ok - a few breakfast items and reasonably good coffee.

I didn't know about the potential for a long line in connections nor about the existence of an arrivals lounge (not at all publicized on the flight).

Overall, my first Aer Lingus experience was ok, decent product in flight (I was on the 757 and snagged a single seat) but I was hoping for a bit better on land. Of course, I should have researched smarter on FlyerTalk and better prepared myself