One bishop will be appointed by Pope Francis to the Diocese of Galway, Kilmacduagh and Kilfenora and the Diocese of Clonfert.
Bishop Michael Duignan of Clonfert and Bishop Brendan Kelly of Galway, Kilmacduagh and Kilfenora currently hold the positions.
The change was relayed to the current bishops by the Pope's representative in Ireland, the Apostolic Nuncio, Archbishop Jude Thaddeus Okolo.
In a joint letter to parishioners about the decision, Bishop Duignan and Bishop Kelly said it was "not an amalgamation" and it would not suppress either diocese.
"Both dioceses will continue to maintain their own integrity and autonomy as is but will work closer together, where possible, through the person and ministry of a single bishop."
The letter goes on to say that as the Holy See continues its consultations on the matter, meetings will be organised "at various levels in each diocese to inform the people of God and talk to them about it".
The bringing of two or more dioceses together is known as in persona episcopi.
The Latin term means "in the person of the bishop" and is used by the Catholic Church to designate the union of two or more dioceses under one bishop.
The diocesan structures and institutions (cathedral churches, curial offices and officials, diocesan pastoral councils, etc), diocesan goods (lands, bank accounts, cultural properties, etc) and juridical competences in Canon Law and Civil Law (trusteeship, charities, etc.) of each of the dioceses are left unaltered.
One bishop exercises the pastoral governance of both dioceses equally but each diocese maintains its identity and handles its own "cultural heritage" as it deems fit.
They keep their own personnel or can share with other dioceses; priests will not normally be asked to minister beyond their own diocese unless by a special request or mandate.
Each diocese handles financial administration independently and makes its own pastoral decisions as usual.
The Diocese of Clonfert covers almost the whole of East Galway, and includes parishes in Co Offaly and Co Roscommon.
The Diocese of Galway, Kilmacduagh and Kilfenora stretches from the city of Galway to north Clare.