My second great grandfather John McKay of Welland (who had immigrated to Canada from Scotland) married a Mary Buchanan of Dunnville in 1873. I have not been able to find much about her family, so if there is any chance she is of your Buchanans please let me know. It’s been years since my research, but my notes say that these Buchanans were Irish, but Methodist, which at the time led me to believe they were probably from Northern Ireland.