Know you are trying to trace the McGowans further back but came across Cecil Reid and Casper (sons of Dr. William) boarding at Campbell College in 1901-
www.census.nationalarchives.ie/pages/1901/Down/Ballyhackmore/Ballycloughan/1222793According to 1911 Census Dr. William was born c1844. Not only is that well before the start of civil registration of births (1864) but since civil registration of non-Catholic marriages started in 1845 it's likely the only source for James McGowan's marriage will be from church records.
However, you are in luck since Dr. Hugh Mullin started collectiong records (and publishing books) years ago and has James McGowan's wife's family tree- NORRIS of Cloyfin:
Margaret (died 1892 aged 92) married James McGowan of Outhill. Family-
1. Mary died 1866 age 26
2. James died 1894 age 57
3. Hugh died 1906 age 71 married Ann Jane McLean, Park (d.1907 age 66)
4. Sarah died 1922 age 90
5.
Dr. McGowan of Maghera6. John Reid
Want more?