The 757 operating DUB-YYZ, 'could' be used on a short European route between 7am and 12.30, to ensure its back in time for its YYZ flight, but the 757 offers no extra capacity over a 320/321, and adds complexity to the operation, as only SNN based cabin crew will operate the 757.
Maintenance for the 75s will be done in SNN, so I'd expect they will operate some SNN-DUB-SNN positioning flights. If the early morning SNN-LHR flight was earlier, the 757 could operate it, but realistically, the only European flying they'll do would be on the Tues/Thurs when the JFK doesn't operate