There is a birth in Ancestry.co.uk which can be found by searching for the name 'Haggo'. In fact it is the only result when 'Haggo' is searched. It is of Stewart Walker on 24 Mar 1880 in Belfast, to parents Stewart Walker and Mother Margaret Mc....Haggo. Clearly something is not readable on the birth record where Ancestry has put the dots. The likelihood is that Margaret's full name is McIlhaggo.
Now there is a Margaret McIlhaggo (the only one I know of) in Belfast who the year before, on 27 Feb 1879, married a Walker, though he was, according to my records, Samuel Walker. There could easily be a mistranscription somewhere of Walker's first name, possibly in the baptism record, or even in my own Marriage Index (!). The marriage was at Trinity Church of Ireland, Belfast and I have for some time thought that this Margaret was the daughter of Samuel and Elizabeth (nee Glass). She was born on 14 May 1854 so would have been 26 when Stewart was born. For the time being, despite the discrepancy between Stewart and Samuel for the name of the father, I'm going to settle for this solution as the 'best possibility' until further evidence persuades me otherwise.