Hack,
On the paid site, Roots Ireland, I found ten children of William and Margaret...all in the RC parish of Manister. The only problem is that I did not find any James...but more about that in a minute.
Here are the children: Patrick (1829), William (1830), Henry (1832), Michael (1834), Bridget (1837), John (1839), Thomas (1841), Michael (1842), John (1845), Margaret (1847).
Where you see two of the same names, that usually means the younger one died as an infant or young child. The first John, 1839, has a sponsor named James Costelloe...it happened sometimes that when the priest entered the names of all the parties, some mistakes were made. So, I thought that John, the infant, and James the sponsor were mixed up. However, if they named another child John, 1845, that means the first John died; so my theory is no good. Do you have a baptism record for James (1837)??
Given that James named his eldest son William, he must be in this family.
Regards,