I checked dohop.com and the cheapest seems to be aer lingus. By going to their website directly, you can find a flight in early July at 749€ which is about CAD950 , assuming you don't need a return ticket. Air Canada is more like CAD1350.
Same, use dohop.com or other search websites. Get ideas about airlines, then go to the airlines websites directly and check there. In my experience, buying directly from the airlines is often cheaper.
Once you are on the website of the airline, put in different options to check. If they offer flexible dates, always check that. Some, mostly low-cost airlines, will sell each ticket separately and then it is cheaper for you to buy only one way if you only need one way. Other airlines give you a cheaper price if you buy a return flight. If that is the case, you can set the return flight that you don't need at any time to see when you get the best price. You can have it months in the future if you want to. If you check Air Canada, it is one of those airlines that they are usually cheaper if you buy a return flight.
Not actually... You will get to know if the fare is refundable as it is usually written in such places if not written you can certainly call the customer care and ask them about the fare is refundable or not and book accordingly.
Also add Via.com as one of the links as they are also cheaper in comparison.