Ancestry.co.uk records the name McIlhago 12 times in total in the 1841, 1891 and 1901 Censuses of Scotland, most of which I had recorded except for George and Isabella at 5 Chapel Bank, New Monkland, Lanark in 1841 when George was a Brick Labourer before he went into the Railway Police Force. A male birth is recorded in 1868 in Belfast under both McIlhago and M'Ilhago, though the spelling that the parents, Samuel and Grace (nee Marrs) usually used was McIlhaga. There are also five McIlhago and three M'Ilhago marriages, all in Ireland, as follows:
1849 Margaret McIlhago, Ballymena 2-61, to George McClean,
1850 Eliza McIlhago, Ballymena 2-87, spouse unknown,
1857 Henry McIlhago, Antrim 1-195, to Agnes Gard(i)ner,
1858 Rachel McIlhago, Ballymena 2-79, to John Francey,
1868 Mary M'Ilhago, Ballymena 16-181, spouse unknown,
1872 Ellen M'Ilhago, Belfast 1-413, to John Hinton,
1873 Jane McIlhago, Belfast 11-366, spouse unknown,
1877 James M'Ilhago, Belfast 1-263, spouse unknown.
One death is recorded, that of Sarah M'Ilhago aged 83 in Ballymena (6-110) which interestingly gives us a birth year of 1781. I have no record of a Sarah born in that year, though of course she may have married into the clan. Finally there were a couple of McIlhago Migration references in 1956, arriving at Greenock, Scotland from Montreal, presumably a visit 'home' by Harry (63) and Sarah (61); also a couple of McIlhago names in the Glasgow Phone Books from 1954 to 1975.