Instrument on Irish euros (4)
I believe the answer is:
harp
'instrument on irish euros' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for harp that I've seen before include "''The ... that once through Tara's halls'' (4)" , "Stick to one theme" , "Instrument (for angels?)" , "Instrument for angel" , "Stringed instrument" .)