CaoilfhionnIt is Welsh, pronounced: KAY-lin.
In Irish Gaelic (NOT Welsh, as the poster suggests), Caoilfhionn is pronounced /KEE lin/, /KWAY lin/ or /KWEE lin/. It is made up of the elements caol meaning "slender" and fionn meaning "white; fair-haired." It is also found as Caoileann, and in its anglicized form, Keelin.
See Also: Keelin, Keelyn
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