The Norman Barony of Erris comprised the parishes of Kilcommon (the largest in Ireland) and adjoining Kilmore - The Mullet and offshore Islands.
The Poor Law Unions were established in the 1840s. Belmullet Union consisted of the parishes of Kilcommon and Kilmore, and were the the subject of Griffith's Valuation of 1855. At that time, Ballycroy was placed in the Newport Union.
Kilcommon was divided into districts - Kilcommon West (present Belmullet RC parish with Muingeroon, and townlands east of the Muhnin/Owenmore River, i.e Geesala and Doohoma), Kilcommon East (rest of Kilcommon and Kiltane) and Ballycroy. These districts became the current Catholic parishes of Kilcommon, Belmullet, Kiltane and Ballycroy, with boundaries fixed in 1873.