What does cabotage have to do with flying between the UK and Ireland?
I think the point is that if you fly LHR-DUB-MAN on EI, for instance, this could be construed as cabotage if you have no stop-over in DUB. This is how the US applies its cabotage rules. If you were to fly BOS-YYZ and then YYZ-ORD on AC both on the same day, US authorities would regard that as BOS-ORD via YYZ and therefore cabotage.
I am not aware of a similar approach having been taken this side of the Atlantic.