Isn't it all about Air Passenger Duty. IIRC, The Republic of Ireland abolished APD, whereas a direct flight to Vancouver from the UK would cost £78/156 dependant on seat pitch, more in 1st. At a stroke, this impacted heavily on the NI International flights, where I think they lost around one million passengers p.a. at a stroke of the Chancellors pen. Same with the US, where RoI even has US Immigration checks prior to boarding.
Obviously only suits those with time to organise low risk connection timings.