I didn't find either of his parents on the 1930 Census.
Try to find an exact date of death for whichever parent you think is connected to your family. An Irish immigrant's obituary sometimes tells at least the county of origin and sometimes even the town.
In 1930, James was a well-to-do banker on Long Island, NY, and his brother Henry S was a building contractor in Beacon, Dutchess County, NY.
Try Dutchess Co first, since that's where the parents were. The father James was also a building contractor, so that alone might have got him a decent writeup when he died.
His mother Mary Toohey's 1910 and 1920 censuses say she was born in New York (state, somewhere, not necessarily NYC.) You will need to look for her parents' deaths if hers is the side of the family you are looking for.
In 1930, the sons said their father had been born in the Irish Free State, so that narrows it down to 26 of the 32 counties, if that is any help.
Another possibility: find a biography of Forrestal himself. A lot of times it will mention where his families were from. I found this one referred to in our library, but look at amazon, there may be more:
Driven Patriot : The Life and Times of James Forrestal
by Hoopes, Townsend
