One or two coming into Irish compound (6)
I believe the answer is:
isomer
'compound' is the definition.
(isomer is a kind of compound)
'one or two coming into irish' is the wordplay.
'one or two' becomes 'some'.
'coming into' is an insertion indicator.
'irish' becomes 'ir' (abbreviation).
'some' going within 'ir' is 'ISOMER'.
(Other definitions for isomer that I've seen before include "Compound sharing a formula but having its own atomic arrangement" , "One of different compounds of same formula" , "Compound with same atoms as another but different structure" , "alternative arrangement of small parts" , "similar substance" .)