Hi Stephen, I have a nephew, Stephen D'Arcy. Don't let the spelling throw you. The Galway Darcy's don't have an ' generally. Do you have in your country a government record office? There may be some marriage registry office where you could locate your g/fathers m/certificate to get the place and date and town where Joseph came from. Also then his fathers name. Any old Bible with notes? Any old siblings of Joseph or your uncles/aunts etc. A Church register of a marriage in NJ?
Any old photos that grandpa had? Letters from his siblings/parents in a shoe box? cemetery inscriptions? Shipping records of Joseph arriving in US? Voting rolls at large State libraries? Good luck Steve and lets know if any of these leads helps. Bernie D'Arcy ex Tipperary