The RC catholic church records for Kilkeevin parish (Castlerea) go back further than most parishes in Co. Roscommon but they only go back to 1804 too late for William.
No Griffin records in the Tithe listings for Kilkeevin parish.
The 1855 Griffiths head of household listing for Kilkeevin parish lists an Owen Griffin in Cloonlatieve townland. Not sure if there is any connection back to William.
There were two 1901 census records in Cloonlatieve with griffin families. The head of households were possibly sons of Owen Griffin from the Griffiths.