Prior to its amalgamation, the diocese was divided into unions (or groups) of parishes.
Achonry Union: • St. George's Church,
Tubbercurry • Rathbarron Church
Aughaval Group: • Holy Trinity Church,
Achill Sound • Christ Church,
Castlebar • St. Thomas' Church, Dugort • St. Thomas' Church, Knappagh • Turlough Church • Holy Trinity Church,
Westport Ballisodare Union: • Holy Trinity Church,
Ballisodare • St. Paul's Church, Colloney • Ballymote, Emlaghfad
Galway & Kilcummin: •
St. Nicholas' Collegiate Church,
Galway • Kilcummin Church,
Oughterard Killala Union: • St. John's Church,
Ballycastle • St. Mary's Church,
Crossmolina • St. Patricks Cathedral,
Killala Kilmoremoy Union: • St. Michael's,
Ballina • Killanley Church,
Castleconnor • St. Anne's
Easkey • Straid Church,
Foxford •
Kilglass Church
Omey Union: • Christ Church,
Clifden • Holy Trinity Church, Errislannan • St. Thomas' Church, Moyard • St. Mary's Church,
Roundstone Skreen Union: • Christ Church, Dromard • St. Mary's Church, Kilmacshalgan •
Skreen Church
Tuam Union: • St. John the Baptist's Church, Aasleagh • St. Mary's Church,
Cong •
St. Mary's Cathedral,
Tuam ==List of bishops==