Finally settles trouble between North and South Irish region (5,4)
I believe the answer is:
nails down
'finally settles' is the definition.
'trouble between north and south irish region' is the wordplay.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'between' indicates putting letters inside.
'north and south' becomes 'ns' (I can't justify this - if you can you should give a lot more credence to this answer).
'irish region' becomes 'down' (County Down).
'ail' inserted within 'ns' is 'nails'.
'nails'+'down'='NAILS DOWN'
(Another definition for nails down that I've seen is " Clearly defines".)