Ancient language of the Celts (4)

I believe the answer is:

erse

(Other definitions for erse that I've seen before include "Celtic tongue" , "Old Scots or Irish Gaelic" , "Kind of Gaelic for seer" , "Name sometimes given to languages of the Celts in Ireland and Scotland" , "Old Gaelic for the seer" .)

I've seen this clue in The Irish Times.

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