Fassadinin (Irish: Fásach an Deighnín, meaning 'Wilderness along the River Dinin'[1][5]), sometimes written Fassadining, is a barony in the north of County Kilkenny, Ireland.

[5] According to O'Heerin's Topographical Poem (1420)[8] at the time of the Norman invasion the area was the territory of the clan called the Ua Braonáin (O'Brennan) of Uí Duach.

[5] "Ui-Duach of Ossory of the warm soil, The fair, wide plane of the Feoir, Not easily passable is the wood of the plain, Its protecting chieftain is O'Broenain."

[7] By the 19th century these were restructured into the baronies of Callan, Crannagh, Fassadinin, Galmoy, Gowran, Ida, Iverk, Kells, Knocktopher, and Shillelogher.

[15] Fassadinin contains the towns of Ballyragget and Castlecomer, and the settlements of Odagh, Clogh, Jenkinstown, Coan, Coolcullen and Coolbaun.

[16] Fassadinin is made up of 19 civil parishes, including Attanagh, Odagh, Castlecomer, Kilmenan, Kilmacar, Kilmademoge, Kilmadum, Coolcraheen, Dysart, Donaghmore, Dunmore, Grangemaccomb, Muckalee, Mayne, Abbeyleix, Mothell, Rathbeagh, Rathaspick and Rosconnell.

[21] In the Gaelic Athletic Association in Kilkenny, St Martin's GAA club has a catchment area which is roughly comprehended by the barony.