Have you tried www.theshipslist.com
a canadian website of passenger records, few from Ireland to Canada, but then passengers had to go by steamship from Quebec to Montreal on their way to Ontario.
I'm not researching the Johnston's - no relation. I do this for fun!
My husbands ancestors the Codd/Code family came from Wicklow to Canada by 1834 and settled in Lanark Co. Upper Canada.
These are the Benjamin (no Hardcastle) Johnston on the website of the History Foundation they start
1800, 1849 Cork; Cavan 1821, 1827, 1835; Wicklow 1839;
1849 Donegal, 1861 Leitrim.