I'm not sure if I understand everything in your post. But, I have an idea that might help.
B/M/D registration was civil after 1863. Since your ancestor was born after this date, the General Register Office likely holds the civil birth rcd. Contact this office at http://www.groireland.iehttp://www.groireland.ie. It either holds the rcd at this website, or can direct you to another site.
As a govt agency, it charges a modest fee to cover its expenses. Probably well worth the price if it will help you break this brick wall.