I suggested that daisynook should send me the certificate in question, and I would resize it so that it can be posted here. She sent me the 1901 marriage certificate and John's 1875 birth certificate. However they are in .pdf format and my photo editor doesn't do .pdfs. (This might also be the reason why RootsChat won't accept it?) So I transcribed both certificates.
MARRIAGE
1901, on the Twenty Second day of April at St Mary’s Chapel, Dundee After Banns according to the Forms of the Roman Catholic Church
(Signed) John Walls, Police Constable, Bachelor, Age 26, Usual residence Mossley, Lancashire, parents John Walls, Police Inspector and Mary Ann McCabe now married to John Boyle, General Labourer, to (Signed) Catherine McIntasney, Jute Spinner, Spinster, age 25, Usual residence 19 James Street, Dundee, parents Bernard McIntasney, Hessian Weaver and Catherine McIntasney, M(aiden) S(urname) McKellosky (Dec(eased))
Married by (signed) Michael Phelan, St Mary’s, Forebank. (Signed) William Walls, Witness, Rose A McIntasney, Witness.
Registered 1901, April 24th, at Dundee, Jas Murray, Registrar.
BIRTH
John Walls, born 1875, May Ninth 8h 50 m AM, 8 Albert Street, Dundee. Male. Parents Henry Walls, Iron Moulder (Jour(neyman)), and Mary Ann Walls, M(aiden) S(urname) McCrudden, (who were married) 1872, Octr 21st, Jarrow, England. Mary Ann Walls ‘x’ her Mark, Mother. David Neish, Registrar, Witness. Registered 1875, May 17th At Lochee, David Neish, Registrar.
What this all means is that the John Walls who married Catherine McIntasney in 1901 is not the one in the birth certificate in 1875 in Dundee, whose parents are completely different.
It also explains the other surnames listed for John Walls in the marriage index - his mother's and his stepfather's surnames as well as his father's.
The John Walls who married Catherine McIntasney was born illegitimate in 1875/1876 and is probably registered as John McCabe.
The transcription of the 1881 census at FindMyPast lists John Walls, grandson, aged 6, in the household of Thomas McCable [sic], aged 48, at 223 Hilltown, Dundee. Mary Ann McCable is aged 26, born Dundee. From the listed siblings of Mary Ann, this is the family of Thomas McCabe and Margaret Culwell, both of whom were born in Ireland. A copy of Bridget McCabe's birth certificate - she was born in Dundee in 1862 - will tell you when and where Thomas and Margaret were married.
Margaret Caldwell or McCabe died in Dundee in 1907, aged 67. Thomas McCabe died in Dundee in 1888 aged 53. Their death certificates should tell you the names of their parents, including their mothers' maiden surnames, if that was known to the informant.
The 1901 census lists in the Police Station, Mossley, Ashton-under-Lyne, Lancashire John Walls, Police Constable, aged 26, born Scotland.
There is a birth of a John McCabe in Dundee in 1875 who could be the John Walls who married Catherine McIntasney.
Mary Anne McCabe married John Boyle in Dundee in 1886.
In 1891 John Walls, 37, Police Constable, is listed in Preston, Lancashire with wife Euphemia, 34 and sons John, 16 and Joseph, 14, all born Scotland.
In 1901 John Walls, Police Inspector, married, aged 47, born Scotland, is a boarder in the household of Ellen Livesey in Preston.
A John Walls and Euphemia Hamilton were married in Glasgow in 1882. If this marriage shows John as a police officer, I think this would nail him as the father-in-law of Catherine McIntasney.