Irishmen found to supply time (6,9)
I believe the answer is:
'time' is the definition.
(I know that time can be written as fourth dimension)
'irishmen found to supply' is the wordplay.
'supply' indicates an anagram (in a supple way).
'irishmenfoundto' anagrammed gives 'FOURTH DIMENSION'.
(Other definitions for fourth dimension that I've seen before include "It may be described as time" , "feature of spacetime" .)