Forth (Irish: Fothairt) is a barony in County Carlow, Ireland.

Forth takes its name from the Irish Fortuatha, a term that described a region (tuath) not ruled by members of the dominant dynasty of a province.

Physical features include the Burren River and Mount Leinster.

Forth barony is bordered to the east by St. Mullin's Upper; to the north by Rathvilly; to the west by Idrone East; to the northwest by Carlow (all the preceding baronies are also in County Carlow); to the northeast by Shillelagh, County Wicklow; and to the southeast by Scarawalsh, County Wexford.

Uí Nualláin (O'Nolan) were rulers of Fothairt Mag Feá.