Form of address used by the courteous Irish (3)
I believe the answer is:
sir
'form of address' is the definition.
(I've seen this in another clue)
'used by the courteous irish' is the wordplay.
'used by' says the answer is hidden in the clue.
'SIR' can be found hidden inside 'courteous irish'.
(Other definitions for sir that I've seen before include "Formal and deferential address to a man" , "Way to address gentleman" , "Title given to baronet" , "Formal term of address" , "Honourable title" .)