I am descended from Ponsonby Goldsbury McMahon, who was born in Louth in 1826.
He emigrated to Australia and married Bridget Egan in Sydney in 1850. He was the son of Mr McMahon ( I suspect that his father’s name was either William or Charles) who was the contractor who built St. Catherine’s Church in Tullamore in Offally circa 1818, and Miss Gouldsbury, the daughter of the Vicar of Tullamore ( I suspect that her name was Catherine).
According to a contemporary source (his wife) Ponsonby G McMahon was educated in Ardee in Louth, and had an uncle in Ardee named John or possibly James.
Based on traditional Irish naming traditions I would suspect that he was neither the only child nor the eldest child of his parents; his grandfather was the Rev Ponsonby Gouldsbury. I am trying to track down this family.
Regards
Ian Gartlan