Serious measure of drink after fun in Ireland (9)
I believe the answer is:
crackdown
'serious measure' is the definition.
'drink after fun in ireland' is the wordplay.
'drink' becomes 'down' (to down something can mean to drink it quickly).
'after' says to put letters next to each other.
'fun in ireland' becomes 'crack' (in Ireland, 'crack' can mean an enjoyable time).
'down' put after 'crack' is 'CRACKDOWN'.
'of' acts as a link.
(Other definitions for crackdown that I've seen before include "Repression" , "Repressive action" , "Draconian measures" , "severe action" , "Crushing measures" .)