The name Brecknock is an anglicized form of Brycheiniog, the Welsh name of the territory of Brychan (whence the alternative form of Brecon), a Goidelic chieftain, who gained possession of the Usk valley in the 5th century.
On the departure of the Romans, the Goidelic hill-tribes, probably with help from Gower and Ireland, seem to have regained possession of the Usk valley under the leadership of a chieftain of their own race, Brychan, who became the ancestor of one of the three chief tribes of hereditary Welsh saints.
Cil, a retreat, said to be akin to the Goidelic kil - Ciliau-Aeron, Cilcennin.
Primitive Celtic split up, as already shown, into two dialects, represented in modern times by two groups of languages - (i) the Goidelic group, comprising Irish, Scottish, Gaelic and Manx.
Some time must have elapsed before any Brythonic people undertook to defy the powerful Goidelic states, as the supremacy of the Brythonic kingdom of Tara does not seem to have been acknowledged before the 4th century of our era.