Fitzpatrick Family

I live in New Zealand, I have an ancestor James Fitzpatrick, born Maryborough (Portlaoise) Laois 01 Oct 1841. His father was Patrick Fitzpatrick and mother Ellen McDonald. He had two sisters Susan Fitzpatrick, and Margaret Fitzpatrick and a brother William, all younger then James. I have a marriage for Margaret to Lewis Byrne, and they had 7 children, However, I cannot prove that it was "my" Margaret. James came to New Zealand with the British army, and returned to NZ after he was discharged. I can find no trace of his parents or siblings. Can you help please? Thanks Heaps LIsa