Going back to the Mullan side for a moment...
To recap. The parents of the James Mullan b. 1906 who married Margaretta Bennett in 1930 were found to be James Mullan and Margaret Dowie (or spelling variants), when they married at Trinity Church of Ireland, Belfast, in 1904, that James' father was recorded as James Mullan a baker, in the 1911 Ireland census, James Mullan and Margaret Dowie and family were living in the townland of Ballyhowne in Carnmoney civil parish
(Elwyn's post at reply #11).
Going back to the 1901 Ireland census, the same James Mullan who went on to marry Margaret Dowie in 1904 was shown as a 17 year old in the household of his parents at house 27 in Hooker Street Belfast, his parents were shown as James (a baker) and Agnes Mullan, also in the household were a Jane and Jemima Mullan, both with a stated birth place of Scotland
(Elwyn's post at reply #13).
So further to those findings, by taking that Hooker Street household forward to the 1911 Ireland census, one finds that James senior (now an unemployed baker) and his wife Agnes Mullan, just like their son and daughter-in-law, were also living in the townland of Ballyhowne in Carnmoney civil parish:
http://census.nationalarchives.ie/pages/1911/Antrim/Carnmoney/Ballyhowne/187610/In the above census return, it says that James and Agnes were married c. 1878. The two Scottish born children were still in the household, except that Jemima was called what I think was meant to be Wilhelmina ('Whillimina'). A quick check of the births index on the 'ScotlandsPeople' website for those names and the birth years indicated by the census returns quickly brings back a Jane Mullan born 1887 and a 'Williamina' Mullan b. 1891, both registered in districts in the greater Glasgow area and both with a mother's maiden name recorded as Galway. Leading on from that, here is the birth registration for a James Mullan, born on 10 May 1883 at 24 McTier Street, Belfast, to a baker called James Mullan and an Agnes Galway:
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1883/02739/2007064.pdfTherefore, it's looking very likely that the James Mullan who married Margaret Dowie in 1904 was the son of James Mullan and Agnes Galway, who were married in Trinity Church of Ireland, Belfast, on 6 July 1878 (the 1911 census was spot on), with the two fathers recorded as James Mullan (not again!) and Isaac Galway, both labourers:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1878/11106/8065231.pdf