The Canadian Embassy is in St.Stephens Green in Dublin. You will find that any of the local banks will have a supply from the staff/visitors to the embassy cashing their $ at the nearest bank.
No no visa required - you've to fill up a landing card when you arrive to give to immigration IIRC but that's it no visa/visa waiver required for Irish passport holders
I wouldn't stress yourself to much trying to find CA$ in advance ... i'd just go to an ATM when you arrive and get them there - generally it's a cheaper way to get foreign currency.