English, from the Greek "pure, holy or chaste". The French variant is accented: Agnés
There are a number of spelling variations found in the 1841 Census. Searching on Agn*, Ang* and Aug* should find all significant ones.
Note that Agneta was used as a Latinised variant of Agnes in many old documents. The above search on Agn* will take account of Latin case endings.
A private correspondent [LG] has noted Agnes being used in formal records for someone whose birth registration [1830, Catholic] was Ann. Other correspondents [AG, CMcB] have reported Agnes and Ann being used interchangeably in official records in Scotland so we have classified them as full synonyms.