There are a few one-offs to me found in Ancestry.co.uk. The first three found concern marriage.
John McIlhagor, 1851 Ballymena 2-103, spouse Mary Stewart. John was in fact son of William and Agnes (nee McCosh) for Ballycloughan, County Antrim and this family (mine!) usually used the spelling McIlhagga. They were married at the 1st Presbyterian Church, Broughshane on 15th July 1851.
The next is particularly interesting as it is from the record of Marriage Banns in Paris, France, 1860-1902. It records the name of Marie Sudour who was to marry a man with the surname M'Ilhague. But when or where I know not. There is one Scottish record of John McIlhague marrying Isabella McCallum. They had a daughter Jean in 1824 in Dundonald, Ayrshire, so could be related to our clan there. Isabella had a brother Alexander who married Elizabeth McElhago, daughter of James McElhago and Jane Harvey.
Third we have David G. McIllhaggow (also McIllHaggow) in a published Family Tree on Ancestry, who was born about 1700 and married Janet G. Murdock on 16 February 1725 in Dalmellington, Ayrshire. The 'G' in Janet's name is new to me. This was David's first marriage. He was I think born in 1702. He and Janet had a daughter Janet in 1732. His second marriage was to Elizabeth Dunbar on 22 March 1740 also in Dalmellington, presumably after the death of Janet. They were to have two children, William and Mary. It is possible that William migrated to Ulster and founded the family in Maxwell's Walls, County Antrim, as William McIllhago. He was born in 1743.