On ancestry.com, there's a Williams Family Tree which says that Henry's father was RICHARD (mother not known). I checked Griffiths Valuation, but the only Richard Justin was in Cork c. 1852 (3 entries). But there is a Richard Justin, aged 87 (so born c. 1814), on the 1901 Irish census living with a son & grandchildren in Queens County. I wonder if this could be Henry's father?
I presume neither of his Civil marriage certs mentions his mother? Any JUSTIN witnesses? Have you tried for a Church record, which sometimes notes the mother. Did he have siblings? If so, perhaps you can get a baptism, birth or marriage cert for them which might list parents.
You may already have this, but just in case ....Here's the marriage of the son, Henry Edward Justin in 1902 (married Annie Trevor):
Ireland, Civil Registration Marriages Index, 1845-1958
Name: Henry Edward Justin
Date of Registration: Apr-May-Jun 1902
Registration district: Mountmellick
FHL Film Number: 101260
Finally, this thread may be of interest http://boards.ancestry.com/thread.aspx?o=0&m=18.104.22.168&am...