The 1861 Canadian Census in Emily, Victoria County, Canada West (now Ontario) shows Catherine Holland, aged 28, living with Cornelius & Catherine McCarthy. This leads me to believe that she was either a widow when she married John Cummings, or she was the daughter of Catherine from a first marriage. Note that there is a Michael Holland, also living with a family. He seems to be married with small children; he is also born in Ireland. There is also an Ellen McCarthy, aged 20, in the household, who is likely a daughter of both Cornelius & Catherine. This suggests a marriage date for Cornelius & Catherine of about 1840.