[1] Garrycastle is located in westernmost County Offaly, on the east bank of the Shannon and containing most of the lower Brosna valley.

Garrycastle was the ancient territory of the Delbna Ethra, represented in later times by the Mac Cochláin (MacCoughlan) sept.

The Colgan family was also centered in this barony, as was the Mac Uallacháin (MacCuolahan) sept of Muintir Cionaetha.

The Ó Maoileoin (O'Malone) sept is cited with early events involving Clonmacnoise in the northwest corner of this barony.

This article related to the geography of County Offaly, Ireland is a stub.