Pope Francis has appointed Bishop Kevin Doran as Bishop of Achonry, while he continues to serve as Bishop of Elphin.
The two Dioceses are now united in persona episcopi, meaning they will be led by one Bishop.
The announcement was made this morning during mass at the Cathedral of the Annunciation of the Blessed Virgin Mary and Saint Nathy in Ballaghaderreen.
This marks the next step in the planned reorganisation in the Province of Tuam.
Reverend Vincent Sherlock, Chancellor of the Diocese of Achonry, says both Dioceses will gradually become one while respecting their distinct cultures and histories.