Catherine Mitchell was married to a Thomas not a John. In the 1871 census she is living in Boundary Place with husband Thomas age 41, a stone mason's labourer, she is 40 and their daughter Mary age 17. All three of them were born in Ireland. Thomas died in Shoreditch in Dec 1880 age 51. Their are also three labourers living with them also born in Ireland.
In 1881 census Catherine age 51 a widow living with niece Mary age 18 at Boundary Place and a couple of Irish labourer lodgers.
So they must have married in Ireland, have not traced it yet.