I suggest getting their death certificates and obituaries. The death certificates should have their parents' names. The obits might list parents and possibly surviving siblings which should make it easier to find them in the census.
Sometimes on the death certificates of a person they list what county or town their parents were born in. Yesterday I found just such a death certificate. Perhaps by getting the death certificates of John and his siblings you might get lucky and it will tell you where in Ireland John's father was from.