The barony of Castleknock is one of the baronies of Ireland. Originally part of the Lordship of Meath, it was then constituted as part of County Dublin. Today, it is in the modern county of Fingal, Ireland. The barony was originally also a feudal title, which became one of the subsidiary titles of the Viscounts Gormanston.

The Lordship of Meath was an extensive seigneurial liberty in medieval Ireland that was awarded to Hugh de Lacy by King Henry II of England by the service of fifty knights and with almost royal authority. The Lordship was roughly co-extensive with the medieval kingdom of Meath. At its greatest extent, it included all of the modern counties of Fingal, Meath, Westmeath as well as parts of counties Cavan, Kildare, Longford, Louth and Offaly. The Lordship or fiefdom was imbued with privileges enjoyed in no other Irish liberty, including the four royal pleas of arson, forestalling, rape, and treasure trove.