A second person for whom the IGI uses the surname spelling McIlhaggar is Eliza Ann who in the 1870s had several children as a single mother, including James (1872), Andrew (1875), William John (1879) and Mary Jane (1879). At present I cannot find a family into which any of these individuals fit. I assume that William John and Mary Jane were twins.
A third 'no family' person is Margaret McIlhago, father Henry a farmer, who married George McClean on 24 May 1849. She was from Kirkinriola, County Antrim, which is just north of Ballymena. George's father was James, a Labourer. Margaret was probably born in the mid 1820s. Again at present I have no positive clue which would enable me to fit Margaret into a wider family. They married at First Presbyterian Church, Ballymena, Margaret's surname is also recorded as McIlhaga. Both she and George gave their occupation as 'Servant'. Their witnesses were John McBride and William Cairns.