I have been looking at this for years, and I wonder if some fresh eyes might shed some light. However please don't spend time looking for the Lacy family except insofar as it sheds light on William Duncan's marriage. It's William who is related to me, and I don't really need to know all about the Lacys other than Mary, if she is his wife.
1851 Census: 21 Moor Street, Blackburn (household of Catherine Conway, lodging house keeper, born Ireland)
Micheal Lacy Lodger Mar 65 Chair bottomer Ireland
Bridgett Lacy Lodger Wid 45 Ireland
Mary Lacy Lodger Unm 17 Drawer at factory Ireland
Ann Lacy Lodger 15 Creeler at factory Ireland
Bridgett Lacy Lodger 13 Scholar Ireland
1861 Census: 101 Penny Street, Blackburn, Lancashire
Michael Lacy Head Mar 56 Chair bottomer Ireland
Bridget Lacy Wife Mar 40 Housekeeper Ireland
Honor Lacy Daughter Unm 20 Cotton spinner Ireland
Mary Lacy Daughter 22 Cotton spinner Ireland
Michael Conway Cousin Unm 13 Scholar Yorkshire, Leeds
Bridget Tyman Visitor Unm 28 Weaver cotton Ireland
1871 Census: 14 Back Banks, Little Bolton, Lancashire
Bridget Lacy Head Mar 56 Smallware dealer Ireland
Maria Riley Daughter 17 Lancashire, Bolton
Mary Flanagan Sister Unm 52 Hawker Ireland
1871 Census: Lancashire Court, Westminster, London
Mary Lacy Head Wid 36 Laundress Ireland
Bridgett Lacy Daughter 10 Scholar Marylebone
James Lacy Son 6 Scholar St George
William Lacy Son 1 St George
1881 Census: 118 Camden Road, Tonbridge, Kent
William Duncan Head Mar 56 Painter and cabinet maker Scotland, Murrayshire
Mary Duncan Wife Mar 45 Blackburn
Bridget Lacy Mother-in-law Wid 66 Lancashire
1891 Census: 67 Beulah Road, Croydon
William Duncan Head Mar 66 General dealer Scotland
Mary Duncan Wife Mar 56 Lancashire
1901 Census: 67 Beulah Road, Croydon
Mary Duncan Head Wid 63 General dealer Blackburn
Kate McLaughlin Visitor Unm 25 Independent means Blackburn
Harriet McLaughlin Visitor Unm 21 Independent means Blackburn
Annie McLaughlin Visitor Unm 19 Independent means Blackburn
Was William Duncan's wife Mary Lacy, and if so where and when were they married? I have William's baptism and death (Croydon, 1898), and reams of information about the rest of his family, but I have not found him in the 1851, 1861 or 1871 census. They were not married in Scotland, and I have not found a marriage in England or Wales, or indeed in Ireland.
Ideas, anyone?