Originally Posted by
Craig6z
Do I need to pickup my luggage at Terminal 1 and recheck it for the Aer Lingus flight?
Yes. You will also need to enter Canada, there is no airside way to connect between T1 (AC) and T3 (EI). So make sure you have the required paperwork to enter the country.
After you reclaim your bag and take it through Canadian customs, you should in principle be able to drop it on the "connections" conveyor belt right away. However, I'm not sure I'd trust the inter-terminal connection, and would probably carry it over to T3 myself and drop it at the EI desk there.