Good question. Aer lingus served Montreal with B707s in the '60s and 70's. Toronto was served with B747s briefly in !981-82. I am open to correction on this but I think both were discontinued due to a combination of high seasonality and low yield. Prior to the ending of the compulsory Shannon stop, the Canadian authorities would not allow Aer Lingus to operate scheduled services into Toronto as retaliation for Air Canada not being allowed to operate direct into and out of Dublin. The Montreal service operated DUB-SNN- Montreal-ORD. A nice day's work for the crew. The Air Canada Toronto- DUB service of recent years used a very small B767 variant and operated only in June-July- August. For the reasons given by a previous poster this would not make sense for Aer Lingus but it does for Air Canada. Hope that helps.