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 here were re-formed into the 3 Western Division. The data for those units is given in the files in the correct sequence for that Division.

1 Battalion: Unit strength on 11 July 1921 was 258 all ranks. On 1 July 1922 this unit was designated as 1 Battalion, North Roscommon Brigade, 3 Western Division. The companies of the 1 Battalion were located as follows: Boyle, Doon, Ballinameen, Beedogue and Frenchpark.
2 Battalion: Unit strength on 11 July 1921 was 142 all ranks. On 1 July 1922 this unit was designated as 2 Battalion, North Roscommon Brigade, 3 Western Division.The companies of the 2 Battalion were located as follows: Ballyroddy & Mantua, Killina, Tulsk, Creeve, Elphin and Aughrim.
3 Battalion: Unit strength on 11 July 1921 was 351 all ranks. On 1 July 1922 this unit was designated as 3 Battalion, North Roscommon Brigade, 3 Western Division.The companies of the 3 Battalion were located as follows: Carniska, Curraghroe, Scramogue, Tarmonbary, Slatta, Ruskey, Kilglass, Kilrustan, Strokestown and Cloonfree.
4 Battalion: Unit strength on 11 July 1921 was 200 all ranks. On 1 July 1922 this unit was designated as 4 Battalion, Arigna Brigade, 3 Western Division.The companies of the 4 Battalion were located as follows: Arigna, Ballyfaran, Crossna, Keadue and Cootehall.
5 Battalion: Unit strength on 11 July 1921 was 155 all ranks. On 1 July 1922 this unit was designated as 4 Battalion, North Roscommon Brigade, 3 Western Division. The companies of the 5 Battalion were located as follows: Kilmore, Drumlion, Croghan, Cootehall and Drumboylan.