Amnesty.co.uk records the registration of 5 McIlhagger and 5 M'Ilhagger marriages. First, McIlhagger:
James, 1853, Ballymena, Vol.1, p.717, spouse Jane Middleton.
I was aware that James' name was sometimes spelled McIlhagger but I think his son Robert consistently spelled it McIlhagga, as did his descendants;
Margaret 1854, Ballymena, Vol. 1, p.760, spouse John Hill.
My understanding is that the 'normal' spelling of Margaret's and her father Crawford's surname was McIlhagga, from tenancy documents;
Mary Kathleen, Oct-Dec 1909, Belfast, Vol. 1, p. 406, spouse James Boyd.
This is the family that normally spells the surname with 'er';
John George, Apr-Jun 1911, Belfast, Vol.1, p.485, spouse Sarah Millar.
Samuel Robert, Apr-Jun 1919, Belfast, Vol.1, p.451, spouse Elizabeth Lewis.
Mary Ann, 1867, Ballymena, Vol. 16, p. 109, spouse ???
This is a new marriage to me. I have no idea of her spouse;
John, 1870, Belfast, Vol. 1, p. 417, spouse Mary Jane Hull.
The normal 'er' family, plus a new registration to me;
George, 1876, Belfast, Vol. 16, p. 485, spouse Mary Jane Boyd.
David, Jul-Sep 1906, Belfast, Vol.1, p.320, spouse Elizabeth Louise Sherwood.
William John, Apr-Jun 1908, Belfast, Vol.1, p.447, spouse Eleanor M'Arthur.
Ditto. However I may have noted this name down incorrectly (or else Amnesty has mistranscribed) because it should be William Boyd McIlhagger.