Allen is a Celtic surname, originating in Ireland, and common in Scotland, Wales and England.
 
It is a variation of the surname MacAllen and may be derived from two separate sources: Ailin, in Irish and Scottish Gaelic, means both "little rock" and "harmony", or it may also be derived from the Celtic Aluinn, which means "handsome". Variant spellings include Alan, Allan, etc.
 
The noble family of this surname, from which a branch went to Portugal, is descended of one Alanus de Buckenhall.Wikipedia