www.lawsons.ca lists an appropriate marriage.
The certificate will be available from the Registrar Genreral in Douglas.

1871, Douglas, district 14, schedule 165, 3 Auckland Grove
Michael Reddington, head, married, 31, broker, England
Sarah, wife, married, 31, Ireland
Ellen Matthews, niece, 10, scholar, Douglas

Douglas, district 4, schedule 4, 1 Lord Street
Patrick Mathews, head, married, 50, labourer, Ireland
Ellen, wife, married, 50, Ireland
Thomas, son, 13, scholar, Douglas

1861, Douglas, district 4, schedule 269, Fancy Street
Patrick Matthews, head, married, 40, labourer, Ireland
Ellen, wife, married, 30, Ireland
Thomas, son, unmarried, 3, Ireland
Ellen, daughter, unmarried, 11, Ireland

I think it is the same family. Possible explanations:
Ellen died shortly after the census and a second daughter was also named Ellen.

There is an enumerator's copying error (the fiche is quite clear at 11). Either the age shoule be 1 or possibly 11 months.