People did have a choice where they went but Canada was often cheaper to get to than the US, and even if bound for the US folk sometimes went via Canada to save money. The main reason for this apparent anomaly was that during the early 1800s the US authorities introduced restrictions on the numbers of passengers that could be carried to prevent over-crowding. This had the consequence of increasing fares to the US, but until Canada introduced similar requirements many years later it remained significantly cheaper to get to.
Migrants often settled where other folk from their neighbourhood had gone. So once there was an established community in Canada who wrote back to say things had worked out fine, it was inevitable that more would follow, so a steady stream of travellers then chose Canada.
I have attached a transcript of a letter from Quebec in 1847 which mentions migration from Sligo, albeit briefly. The original is in PRONI in Belfast.