Irishman on church plot (5)
I believe the answer is:
patch
'plot' is the definition.
('patch' can be a synonym of 'plot')
'irishman on church' is the wordplay.
'irishman' becomes 'pat'.
'on' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'pat'+'ch'='PATCH'
(Other definitions for patch that I've seen before include "Material to cover repair" , "Court plaster" , "Plot - piece of cloth - period" , "Mend with extra piece" , "Piece of cloth put on another" .)