English, from Hebrew Hanna/Hannah, "favoured one". ONC suggests that the primary spelling should be Anne.
Ann is also a Jewish diminutive for Hanna/Hannah.
A private communication [LG] records Agnes being used in formal records for someone whose birth registration [1830, Catholic] was Ann.
Another correspondent [CMcB] has noted a baptismal Agnes shown as Ann in official records in Kincardineshire, Scotland. These two names were considered to be fully synonymous in the 17th century and earlier and this has continued in some areas until recently.
The Clan MacLean website suggests that Una is the Gaelic equivalent of Ann but this is not suported by other sources.