158th Infantry Brigade arrived in Northern Ireland in October 1939. This brigade included 4th Battalion, 6th Battalion, and 7th Battalion Royal Welch Fusiliers. Also attached from 160th Infantry Brigade was 2nd Battalion Monmouthshire Regiment.
On arrival, 158th Infantry Brigade established its headquarters in Lisburn, Co. Antrim. 160th Field Ambulance also set up in the Co. Antrim town. 4th Battalion Royal Welch Fusiliers was in Belfast, Co. Antrim. 6th Battalion Royal Welch Fusiliers was in Lisburn, Co. Antrim. 7th Battalion Royal Welch Fusiliers was in Derry/Londonderry. 2nd Battalion Monmouthshire Regiment was in Ballymena, Co. Antrim.