Its hard to judge how it may work out however two carriers is a big ask considering for years nobody wanted it year round.
Aer Lingus have the better cost base/aircraft and feed to the route. They do cut to 3 weekly for most of Q1 2015 but Air Canads have lots of numbers on % of pax connecting at LHR so with that in mind they must think a year round service is viable however EI may not be part of the plan....
If it doesn't work for both WJ will have no problem making it work.