I think Aer Lingus/EI and BA flights to the Republic all fly from an annex in the south terminal not the north (although this may change in the future)

It's a special little annex for flights departing within the British Isles but outside the UK(Rep of Ireland, Channel Isles, IOM)- these countries share immigration with the UK but not customs so once you have entered one you have entered them all.

Sometimes staff at Gatwick get confused and list these flight on the domestic monitors until they realise their mistake and they jump to the international section in the last five minutes.

Flights are BAe 146s (as are BA's ) - possibly not the most comfortable in the world.

EI may start flights to Dallas now they are in OW - them seem to be planning to join AA in San Jose at least