Hello Jap,
You might be right, but about James McGranathan & Jane Brooks mariage, I found some time ago a mariage between Jane Brooks & James Migranscher, in 1854, in New Monkland or Airdrie!
I think that’s my ancestors, because the dates and place corresponds and the two surnames McGranathan & Migranscher are not that different! The “Mc” became “Mi”, the “Gran” stayed the same, and the “-athan” became “-scher”. I don’t know how come it changed, but everything is possible!
My Irish ancestors came from Northern Ireland, so I guess they were Protestant, that would explain that I don’t have problems finding them in the Scottish records!
By the way, Jane Brooks could have been born Jean or even Janet! I think you were aware about that, but in case you didn't know.

Of course I can give you all I have on the Family.
* Death certificate of James McGranathan :
James McGranathan, stocker, married to Jane Brooks, died on November 11, 1894 at the Royal Infirmary in Glasgow, his usual residence was 12, Strachebede (not sure if it is exactly was it is written) Street.
He died at the age of 60 years old. Parents were John McGranathan (deceased) , General Labourer & MaryAnn McKay (deceased). The informant of the death was James McGranathan, his son.
* Death certificate of Jane Brooks :
Jane McGranthin, widow of James McGranthin, Labourer, died on June 23, 1902 in Glasgow at 250, Humleddem (not sure) Drive.
She died at the age of 64 years old. Parents were Walter Brooks (deceased) , Hand Loom Weaver, & Janet Scott (deceased). The informant of the death was Mary Miller, daughter, present.
*
1871 Census :
Given name : James Janet Mary John Janet James Walter Sarah
Surname : McGrannathan " " " " " " "
Relation to head : Head Wife Daughter Son Daughter Son Daughter Daughter
Condition : Married Married Unmarried
Age : 45 34 15 13 11 6 3 1
Sex : M F F M F M M F
Occupation : Fireman Mill Worker Mill Worker Scholar
Birthplace : Ireland Glasgow Rothesay Shotts Shotts Penicuik Glasgow Shotts
*
1881 Census :
Given name : Edward Jane Jane Sarah Walter John
Surname : McGranthin McGranthin McGranthin McGranthin McGranthin Sutherland
Relation to head : Son Head Daughter Daughter Son Grandson
Condition : N/A Married Unmarried N/A N/A N/A
Age : 9 42 20 11 13 5 month
Sex : M F F F M M
Occupation : Scholar Mill Worker Mill Worker Scholar Paper Maker
Birthplace : Linlithgow Glasgow Shotts Shotts Glasgow Milngavie
*
1891 Census :
Given name : James
Surname : McGranathan
Relation to head : Board
Condition : Married
Age : 56
Sex : M
Occupation : Fireman Stationary (it says he was employed by “Boiler”)
Birthplace : Ireland
*
1891 Census :
Given name : Jane Edward John
Surname : McGranthan McGranthan Brooks
Relation to head : Head Son Grandson
Condition :
Age : 33 19 10
Sex : F M M
Occupation :Formerly Paper Mill Worker Paper Mill Worker Scholar
Birthplace : Glasgow Linlithgow Milngavie
This census is wrong! Jane McGranthan was my great great grandmother and the Edward listed as her son was her brother, just have to look at where he was born and when and it is exactly the same as his brother.
The John Brooks isn’t her grandson, because the ages doesn’t correspond and at that time Jane just had her first child, so she couldn’t be grandmother yet!
I don’t know how come they are all living in the same house, where are the parents of Jane & Edward ?
Who is John Brooks ? Strangly his age, place of birth and name are the same as the John Sutherland in the 1881 census !!! i wonder if it isn't the same child

I’ve give you all the main informations I’ve got, I hope you understand everything I gave you!
Thank you!
Best Regards.
Nick.