Thanks guys.
zag -- would you believe I'd already backtracked from the airport web-site and looked at the regional flights out of St. Johns with other airlines. Nothing seemed to work out cheaper or more convenient than AC via the routes I mentioned above, so I was just checking in case someone was aware of a particulary good route.
EvilDoctorK -- If I go via Halifax on the way out I can arrive in St Johns at a civilised hour, at the cost of a longer journey. The weird arrival time on the way back actually suits me as I can sleep overnight in Heathrow and be ready for work in Dublin the next day (all other options involve flying overnight, and I'm not built for sleeping on planes!). However, I'd missed the size of the plane on that route, so that may make me reconsider.