Looking for Davis Ancestry of Ireland USA South Africa

Taking a flier. I am trying to trace my ancestors on my mother side of the family. In particular my grandfather "Robert Edward Davis". All I know is that he was born in Ireland (possibly Tipperary). His family then immigrated to America where he was "shanghaied" as a boy onto a merchant navy ship. He ended up in South Africa, where he raised a family. He passed on in the early 1960's. Any hints on how I should proceed with my investigation would be appreciated.