There is a tree on Ancestry with a Thomas McQuillan (1841-1906) married to Margaret Graham (1839-1889), and they had a child Helen McQuillan born at Denny, 25 Jul 1880, who died on 18th July 1915 at Stirling.
She had married a William Wiseman, b 1888 and had a child named Margaret in 1904.
I searched further (using Ancestry) and found an interesting connection with Avondale... On the 1901 census at Avondale, there are two famlies of Wiseman's on the same page. The first one is the interesting one...
...Maggie Wiseman, 45, Auctioneer
James, 24
John, 21
Robert, 15
William, 13
Charles, 10
Christine, 4
Maggie McQuillan, Servant, 19, born Denny.
Now, according to the tree I saw, the William Wiseman that Helen McQuillan married was born in 1888. ...The William Wiseman on the above census was aged 13, so that makes him born in 1888. So could it be that Helen was married to this William Wiseman, lived at Strathaven and then to complicate matters further had the illegitimate John McQuillan in 1907?
Also, there is a Maggie McQuillan, living there as a servant. Was she possibly Helen's sister? She was born in Denny.
Apologies if this is all a wild goose chase. Either way, it is complicated!