You should hope that they came through the U.S. The two main repositories for U.S. immigrant records are at ellisisland.org and castlegarden.org. There are likely similar repositories for other major ports such as Philadelphia, Boston, and New Orleans.
Sadly, the Canadians didn't keep records or even passenger lists. They regarded Irish immigrants as merely British subjects moving from one part of the empire to another. With luck, you might find them in one of the Canadian censuses which began in 1851.