If it's of any help, I looked up the pre-1855 Monumental Inscriptions book for Killearn Churchyard and the following McLays are listed on stones there:
1. James McLay died 25th? August 182(1?) age 35
2. Moses McLay (in Ballewan, Strath) died 24th February 186? age 67
A son who died in infancy
Wife Agnes Brown died 14th February 1869 age 46
James McLay died at Clachanry 12th January 1860 age 66
James' wife Ann Aitken died New Zealand 4th December 1869 age 68
3. [no dates listed] Erected by William McLay, Merchant, Glasgow (in memory of)
His mother Ann Ferguson of Cowden Cottage, Ballikinrain
His sister Margaret died in infancy
His grandfather William McLay, Wood Merchant, Tomhoolie
His grandmother Agnes Yuill McLay
His uncle Archibald McLay
His father James McLay, Wood Merchant, Cowden
His uncles John & Andrew
(Father & two uncles buried in Old Kilpatrick)
4. William McLay, Merchant, Glasgow, died at Bridge of Allan 5th April 1892 age 81
(Founder & president for 24 years of Glasgow Working Man's Evangelistic Assocation)
Wife Helen Smith died 10th March 1847 age 34
Daughter Mary Mackie died in infancy
Son John F., M.D., died 7th May 1867 age 23
Youngest daughter Agnes Jamieson died at Crossmyloof 10th December 1903 age 58 (wife of John Taylor)
Anne