Irish girl meeting conservationists, a thorn in the flesh (8)
I believe the answer is:
irritant
'a thorn in the flesh' is the definition.
(a thorn in the flesh is an annoyance)
'irish girl meeting conservationists' is the wordplay.
'irish' becomes 'IR' (abbreviation for Irish).
'girl' becomes 'rita' (woman's name).
'meeting' means one lot of letters go next to another.
'conservationists' becomes 'NT' (abbreviation for National Trust).
'ir'+'rita'+'nt'='IRRITANT'
(Other definitions for irritant that I've seen before include "Source of continual annoyance" , "Annoying thing" , "Something that annoys, a nuisance" , "Annoying factor" , "distraction" .)