Hello Joe,
I have checked for a marriage on Find My Past and Roots Ireland but have not been able to locate a marriage in Ireland for John and Mary. The Census transcription on Find My Past (a subscription website) reads that John, Mary and family are living with Mary's "Mother" Catherine Keenan in Scotland in 1861 not her Aunt.
1861 Scotland Census, Gallowgate, St Johns, High Church, Lanarkshire, Scotland.
Catherine Keenan, Head, Widow, age 50, born about 1811, in Ireland, a House Keeper.
Anne Mullin, Daughter, Widow, age 26, born about 1835, in Ireland, a Old Clothes Dealer.
Mary McAtee, Daughter, Married, age 22, born about 1839, in Ireland, a House Keeper's Daughter.
John McAtee, Son-In-Law, Married, age 30, born about 1831, in Ireland, a Labourer.
Owen McAtee, Grand-Son, age 4, born about 1857, in Ireland.
John McAtee Jr., Grand-Son, age 2, born about 1859 in Ireland.
1871 Scotland Census, Gallowgate Back Land, 335, St John Glasgow, High Church, Lanarkshire, Scotland.
John McIntee, Head, Married, age 42, a Labourer, born in Ireland.
Mary McIntee, Wife, Married, age 39, born in Ireland.
Owen McIntee, Son, age 13, a Tobacco Boy, born in Ireland.
John McIntee, Son, age 11, a Tobacco Boy, born in Ireland.
David McIntee, Son, age 7, born in Ireland.
James McIntee, Son, age 4, born in Glasgow, Lanarkshire, Scotland.
Catherine McIntee, Daughter, age 2, born in Glasgow, Lanarkshire, Scotland.
1881 Scotland Census, 16 Kirk Street, Barony, Calton, Lanarkshire, Scotland.
John McAttea, Head, Married, age 50, a Labourer, born in Ireland.
Owen McAttea, Son, Unmarried, age 21, a Carter, born in Ireland.
David McAttea, Son, Unmarried, age 18, a Carter, born in Ireland.
James McAttea, Son, age 14, a Carters Boy, born Glasgow, Lanarkshire, Scotland.
Annie McAttea, Daughter, age 10, born in Glasgow, Lanarkshire, Scotland.
David Mcateer
Baptised: 2 Feb 1862
Parish of Dromore, Co. Down
Father: John Mcateer
Mother: Mary Keenan
Sponsor: Catherine Keenan
Mary Mcateer
Baptised: 25 Nov 1860
Parish of Ballymore, Co. Armagh
Father: John Mcateer
Mother: Mary Keenan
Sponsors: Tom Campbell and Mary O'Heir
Catherine McAleer
Baptised: 10 Sep 1848
Donagh Parish, Co. Monaghan
Father: John McAleer
Mother: Mary Kenan
Sponsors: John Magir and Mary Woods
Scottish Births for John AcAtee and Mary Keenan:
MCTEE JAMES, Year 1866, Ref: 644/4 769, District: Calton (Glasgow)
MCATEE CATHERINE, Year 1868, Ref: 644/2 1946, District: High Church
MCATEE ANN, Year 1871, Ref: 644/2 955, District: High Church
MCATEE EDWARD, Year 1873, Ref: 644/4 621, District: Calton (Glasgow)
MCATEE ELIZABETH, Year 1874, Ref: 644/4 1240, District: Calton (Glasgow)
Scottish Deaths:
MCATEE EDWARD, age 4, Mother's Maiden Name KEENAN, Year 1868, Ref: 644/2 1633, District: High Church
MCATEE JOHN, age 51, Year 1888, Ref: 639/ 74, District: Dalziel
Source: https://www.scotlandspeople.gov.uk/
It appears that the McAtee's remained in Scotland and an excellent source of information for Scottish Genealogy is ScotlandsPeople, the indexes are free to search, however if you wish to obtain images you will need to purchase them.
https://www.scotlandspeople.gov.uk/guides/family-history#:~:text=Scotla….
Should I find anything else I will attach at the end of this post.
Maggie McKenna, Carrickmacross