This brigade in the unit formation listed here existed on the first critical date 11 July 1921 only. Prior to the second critical date of 1 July 1922 a number of units listed were re-formed into the 3 Western Division. 1 Battalion: Unit strength on 11 July 1921 was 745 all ranks. On 1 July 1922 this unit was designated as 1 Battalion, East Mayo Brigade, Western Division. The companies of the 1 Battalion were located as follows: Swinford, Barnalyra, Charlestown, Tumgesh, Killasser, Meelick and Tullinacurra.2 Battalion: Unit strength on 11 July 1921 was 919 all ranks. On 1 July 1922 this unit was designated as 2 Battalion, East Mayo Brigade, 3rd. Western Division.The companies of the 2 Battalion were located as follows: Carracastle, Derrynacarta, Roosky, Doocastle, Lavey, Cloontia, Brusna, Sinolane, Tibokine, Aughalistia, Liscaul and Edmondstown.3 Battalion: Unit strength on 11 July 1921 was 377 all ranks. On 1 July 1922 this unit was designated as 3 Battalion, East Mayo Brigade, 3rd. Western Division.The companies of the 3 Battalion were located as follows: Kiltimagh, Bohola, Ballinamore, Kilkelly, Aghalestia and Freenegleragh.4 Battalion: Unit strength on 11 July 1921 was 317 all ranks. By 1 July 1922 this unit was divided to form the 3 and 5 attalions, South Mayo Brigade.The companies of the 4 Battalion were located as follows: Bekan, Loughboy, Knock, Crossard, Brackloon, Aghamore, Cloonafarna and Ballyhaunis.