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 310 all ranks. On 1 July 1922 this unit was designated as 1 Battalion, North Sligo Brigade, 3rd. Western Division. The companies of the 1 Battalion were located as follows: Sligo Town, Calry, Cararoe, and Kilmacowen.
2 Battalion: Unit strength on 11 July 1921 was 259 all ranks. On 1 July 1922 this unit was designated as 2 Battalion, North Sligo Brigade, 3 Western Division. The companies of the 2 Battalion were located as follows: Grange, Drumcliffe, Magherow, Ballintrillick, Glenade, Rosses Point and Cliffoney.
3 Battalion: Unit strength on 11 July 1921 was 605 all ranks. On 1 July 1922 this unit was designated as 3 Battalion, South Sligo Brigade, 3 Western Division. The companies of the 3 Battalion were located as follows: Ballymote, Keash, Culfadda, Kilavil, Bunnmadden, Emlaughton, Kilcreevan, Ballinafad, and Derroon.
4 Battalion: Unit strength on 11 July 1921 was 461 all ranks. On 1 July 1922 this unit was designated as 4 Battalion, South Sligo Brigade, 3 Western Division. The companies of the 4 Battalion were located as follows: Gurteen, Cloonloo, Kilaraght, Monasteraden, Edmondstown, and Towninadden.
5 Battalion: Unit strength on 11 July 1921 was 638 all ranks. On 1 July 1922 this unit was designated as 2 Battalion, Arigna Brigade, 3 Western Division. The companies of the 5 Battalion were located as follows: Riverstown, St. James’ Well (also known as Curry), Gleann, Geevagh, Ballyrush, Highwood, Sooey, and Glen Arigna.
6 Battalion: Unit strength on 11 July 1921 was 826 all ranks. On 1 July 1922 this unit was designated as 1 Battalion, South Sligo Brigade, 3 Western Division. The companies of the 6 Battalion were located as follows: Tubbercurry, Cloonaughill, Cully, Curry, Cashill, Cloonacool, Tourlestrane, Aclare, Mullinabreena, Achonry, Moylough, and Glenesk.
7 Battalion: Unit strength on 11 July 1921 was 335 all ranks. On 1 July 1922 this unit was designated as 2 Battalion, South Sligo Brigade, 3 Western Division. The companies of the 7 Battalion were located as follows: Collooney, Coolaney, Ballisodare, Carrick Bannagher, Ballygawley, Ballinacarrow, Screen & Dromard, and Templeboy.
8 Battalion: Unit strength on 11 July 1921 was 350 all ranks. On 1 July 1922 this unit was designated as 1 Battalion, Arigna Brigade, 3 Western Division. The companies of the 8 Battalion were located as follows: Ballintogher, Dromahair, Newtownmanor, Ballinagar, Greaghnafarna, Killavoggy, and Glenn.
9 Battalion: Unit strength on 11 July 1921 was 266 all ranks. On 1 July 1922 this unit was designated as 4 Battalion (Manorhamilton), 3 Western Division. The companies of the 9 Battalion were located as follows: Killargue, Glencar, Ballaghameehan, Duagh, Mullies, Manorhamilton, Glenfarne, and Kiltyclogher.
10 Battalion: Unit strength on 11 July 1921 was 167 all ranks. On 1 July 1922 this unit was designated as 3 Battalion, (Bundoran), 3 Western Division.The companies of the 10 Battalion were located as follows: Bundoran, Kinlough, and Tullaghan.