The marriage likely took place before 1866 based on the births of the children I've found on LDS site:
1. Mary Jane born 26 July 1866 Cromac district, Co.Antrim
2. William born 25 Sept.1867 Belfast
3. William John born 12 May 1869 Belfast
4. James born 11 Dec.1872 Belfast
**
5. Mary Ann born 23 Nov.1876 (address 44 Bank La., Belfast)
6. Elizabeth born 13 Aug.1879 Belfast
** wonder if William McCall and Margaret O'Hanlon moved over to Scotland for a short time? there's a birth for a John Martin McCall 10 Oct.1874 Greenock who's parents have same names. If this is a child of your William and Margaret his birth certificate should list date and place of parents' marriage.
You haven't mentioned what religion the family were but registration of Catholic marriages only started in 1864 but non-Catholic marriages (including those in a Registry Office) started 1845.
