Parish Ahascragh, Townland Kilglass, in the Union of Mountbellow, Galway.
Griffith Valuation of Galway 1856:
There are 11 Nolan/Nowlan's in the Parish, but only 3 in the Townland of Kilgass.
John, James, Thomas all spelled Nowlan
(these might/should be the parents of John Nowlan
If you know the approximate year that John was born, I can look and see who his parents might of been if he was born between 1864-1875. There are about 15 John Nolan's born in those 10 years. All spelled Nolan.
The names of John and Marguerite's children could help point the way to the father's name. Is the first son named after John or?, 2nd. son should be named after Marguerite's father.
They do not appear to be in the 1901/1911 Censuswww.census.nationalarchives.ie
as Nowlan and although there were 49 John Nolan's there is no Marguerite and no Nolan in Kilglass townland.
John may have been from another parish as generally marriages happened in the bride's parish.
Did they move away?
Can't do more without more information about the children and John's approx. birth year.
The telephone directory for Galway has only 0 Nowland's but
but there are some in close counties. There are many more Nolan's. There is 1 Nolan listed as in Mountbellew, Ballinasloe.
There are 6 more in Ballinasloe. You could write to a few in that area - addresses are sufficient for mail delivery.