I was looking up the mannix board when I happened to read your inquiry.
I have a Maurice Mannix whom married a Margaret Dowd on my family tree. Alas all the info I have on their children is a James Mannix born about 1834 County Kerry Ireland. He married a Joahanna Finnaughty 29/9/1856 In Tralee Ireland.
Both immigrated to Australia. Maurice would be my Great great great grandfather. Hope that it of some help would love to hear from you if it is.