Both John & Callaghan are extremely common names in Ireland. Civil registration didn't start until 1864 so you will have to rely on church records. Firstly church records are far from complete, secondly it will be impossible to identify the correct John Callaghan without knowing where in Ireland he came from. Do you have a marriage cert? It would give father's name. Or a death cert? Some countries give good genealogical information on death certs.
You need to dig deeper wherever he lived and died.
