They are used to enlarge Irish butter factories (8)
I believe the answer is:
implants
'they are used to enlarge irish' is the definition.
The definition suggests an adverb but the answer is not.
'butter factories' is the wordplay.
'butter' becomes 'im' (I can't justify this - if you can you should give a lot more credence to this answer).
'factories' becomes 'plants' (manufacturing plants).
'im'+'plants'='IMPLANTS'
Can you help me to learn more?
(Other definitions for implants that I've seen before include "Inserts, ideas in mind or substance into living tissue" , "bust up thanks to these" , "grafts" .)