Scottish and, occasionally, Irish. Ewen is sometimes found in the 1841 Census of Aberdeenshire where it is more frequent than Ewan [q.v.]. See also Euan.
In old documents in Latin, Eugenius is used for Euan/Ewan/Ewen as well as for Eugene. Care must be taken when searching to allow for Latin case endings.
One correspondent [J-AC] records a family over several generations using Hugh, Hew and Ewen interchangeably in Argyll, Scotland.
According to the Clan MacLean Website, Hùisdean or Ùisdean is the Gaelic equivalent of Hugh, Ewen and Austin.