Another frith place name is Skenfrith on the welsh borders with a castle built by the marcher Lords after the Norman conquest. In this case it is an anglicised version of the welsh Ynysgynwraidd, which means "island of Cynfraeth", possibly a local 6th century leader.

How Skenfrith derives from this escapes me as the approximate pronunciation of the welsh is "innisgunwraeth" (apologies to native welsh speakers)