There is only one Dennis Cummins in Wexford in the Tithe Applotment Book dated 1826 in Ratthurlin, Killegney, Co. Wexford.
On board the Gertrude which arrived on 23 February 1849, are Mrs. U. Cummins, aged 40; Denis Cummins, aged 40; Michael, aged 4; Ann, born at sea. This does not seem to match the info you have for your family.
Seems to me that if the marriage record is worded the way you have written it, that Dennis & Mary did not immigrate, but rather, stayed in Ireland.
Dennis Joseph died in British Columbia:
Name: Dennis Joseph Cummins Event Type: Death Event Date: 04 Sep 1917 Event Place: New Westminster, British Columbia, Canada Gender: Male Age: 40 Marital Status: Single Birth Date: 21 May 1869 Birthplace: , Ontario Father's Name: John Cummins Mother's Name: Catherine Holland