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 South Mayo Brigade, 2 Western Division.
1 Battalion: Unit strength on 11 July 1921 was 534 all ranks. The companies of the 1 Battalion were located as follows: Cross, Neale, Clonbur, Kilmaine, Shrule, Carras, and Thomastown.
2 Battalion: Unit strength on 11 July 1921 was 408 all ranks. The companies of the 2 Battalion were located as follows: Balinrobe, Cloonacastle, Robeen, Partry, Srah, Loughmask, and Hollymount.
3 Battalion: Unit strength on 11 July 1921 was 228 all ranks. The companies of the 3 Battalion were located as follows: Claremorris, Ballyglass, Irishtown, Carramore, Ballindine, Kilcoman, Murneen, Crossboyne, and Ballygown.
4 Battalion: Unit strength on 11 July 1921 was 327 all ranks.The companies of the 4 Battalion were located as follows: Balla, Ballyglass, Belcarra, Mayo Abbey, and Tavanagh.