Decies-within-Drum (/ˈdeɪʃə/; Irish: Na Déise laistigh den Drom[1]) is a historical barony in County Waterford, Ireland.

[2] Decies (Déisi Muman) bounded in by the Drum Hills (Drom Fhinín).

Decies-within-Drum is located in the southwest of County Waterford, east of the Munster Blackwater and including the coast between Youghal and Dungarvan harbours.

[3] Déisi Muman were an ancient Gaelic Irish tribe that occupied this territory; their name means "Vassals of Mumu."

[4] Decies-within-Drum was anciently the territory of the Ó Broic, a surname literally meaning "descendant of badger", perhaps a totemic animal.