Study a king from Ireland having head on Irish and old Roman coins (7)
I believe the answer is:
denarii
'roman coins' is the definition.
(I've seen this before)
'study a king from ireland' is the wordplay.
'study' becomes 'den' (I've seen this before).
'king' becomes 'r' (r is abbreviation for rex).
'from' means one lot of letters go next to another.
'ireland' becomes 'ii' (I can't explain this - if you can you should believe this answer much more).
'den'+'a'+'r'+'ii'='DENARII'
'having head on irish and old' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for denarii that I've seen before include "22 [COINS]" , "Silver coins of ancient Rome" , "A lot of asses" , "Old coins" , "Bits" .)