After Air Canada expanded their Dublin to Vancouver service late last year, they are now offering a very good sale on this route!
Many different non-stop return options are now available in September and October 2017 for well under 300 EUR roundtrip prices. There are also still options available in August (which is just in a few weeks time) for about 50 EUR more.
You can find a complete overview of the available prices, travel dates and booking options through this✈ link.
Also through Kayak there are other – even cheaper – options with a transfer in London, from 255 EUR return: