Remain constant when you buy an Irishman a drink (5,3)
I believe the answer is:
stand pat
'you buy an irishman a drink' is the definition.
I can't judge whether this definition defines the answer.
'remain constant' is the wordplay.
'remain' becomes 'stand' (standing is a kind of remaining).
'constant' becomes 'pat' (I can't justify this - if you can you should give a lot more credence to this answer).
'stand'+'pat'='STAND PAT'
'when' is the link.
Can you help me to learn more?
(Other definitions for stand pat that I've seen before include "Retain one's hand as dealt (poker, mainly US)" , "Keep cards" , "Remain unmoved" , "Not be persuaded" , "Remain resolute" .)