Thank you for your prompt replies wimbush75 and ~Merlin~
Michael's birth certificate indicates under Issue: 2 males living;
William and Mary appear to have only one possible previous child registered in NSW which was MARTHA ( V1840338 145/1840). Michael may have been the second child, although I found no registry entry for his death, with the parents stated.
They may have immigrated before 1846 and had other issue. I have found three possible age correlations:
1. Michael EGAN 35, wife Bridget 38, John 5, Michael 3, Mary A (baby) arrived Port Adelaide on 10-10-1849 on Cheapside from London. (Possibly the SA branch of the EGAN name)
3. Michael arriving on Abysinnian from Tipperary on Sept 20 1859. (Possibly the QLD branch of the EGAN name)
2. I have elimated Michael Egan aged 3 arrived aboard the Harry Lorrequer, on March 14, 1851. This family settled in Victoria.
Paul