Thanks c-w-s, that's really helpful to know.

Can't say I'm tempted by your final suggestion of using left luggage in LHR arrivals however; once I'm in arrivals, landside, I reckon I would be up to be car (pre-booked in the T5 short-stay car park), grab my bag, and be back into T5 departures in less than 10 minutes. I would anticipate it taking me at least that long to reclaim my bag from left luggage, especially if there were even one person in front of me in the queue.

But you've convinced me this is a bad idea; I'll take my bag to Dublin after all. And try to remember to book a longer connection time when I book my second exEU.