Precious gem setter will mention in poetic description of Ireland (7,4)
I believe the answer is:
emerald isle
'ireland' is the definition.
(synonyms)
'precious gem setter will mention' is the wordplay.
'precious gem' becomes 'emerald' (I've seen this in another clue).
'setter will' becomes 'ill' (I'll, i.e. 'the person writing the crossword will').
'mention' shows a homophone (sound like) (I've seen 'mentioned' mean this).
'ill' is a homophone of 'isle'.
'emerald'+'isle'='EMERALD ISLE'
'in poetic description of' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for emerald isle that I've seen before include "jewel of a place" , "Name for Ireland" , "divided country" , "where one might get whiskey" .)