Knock Irishman over (3)
I believe the answer is:
tap
'knock' is the definition.
(I know that knock can be written as tap)
'irishman over' is the wordplay.
'irishman' becomes 'pat'.
'over' says the letters should be written backwards.
'pat' reversed gives 'TAP'.
(Other definitions for tap that I've seen before include "Water source; take advantage of" , "It's called a faucet in America" , "non-violent strike" , "Dancing" , "that's less hard than 11 [BASH] or 22 [BATTER]" .)