Without a doubt Irish men going to Spain certain to succeed (8)
I believe the answer is:
surefire
'certain to succeed' is the definition.
(I've seen this before)
'without a doubt irish men going to spain' is the wordplay.
'without a doubt' becomes 'sure' (I've seen this in another clue).
'irish men' becomes 'fir' ('men' in Irish Gaelic).
'going to' says to put letters next to each other.
'spain' becomes 'e' (abbreviation for Espana).
'sure'+'fir'+'e'='SUREFIRE'
(Other definitions for surefire that I've seen before include "certain to succeed" , "Always effective" .)