Paul:
Roots Ireland only has the baptismal records for Thomas and Mary. There is a gap in Castlerea baptismal records from May 1809 to January 1816 https://registers.nli.ie/parishes/0551 ; so it is possible that there were younger children. I did find a June 7 1816 baptismal record for a Michael Costello at Castlerea. The father was Michael Costello but the mother was shown as Eleanor Collary. Could be the same couple.
By the way, the parents (Michael Costello and Mary Colery) were married February 15 1806 and Thomas was born in December 1806.
Civil registration of deaths started in 1864. Castlerea RC parish (see link above) has death info for 1816-1819 and 1852-1855.
I looked for death records for Michael Costello in the Castlerea registration district up to 1880 and did not find a good lead.
I also looked for a Mary Costello record and there was a Mary Costello who died in 1869 at age 84 but she died in Kiltullagh parish so I think this record was for a different Mary Costello.
Roger McDonnell
