One who revels in Irish republic's model meets king and queen (9)
I believe the answer is:
roisterer
'one who revels' is the definition.
(to roister is to party or celebrate)
'irish republic's model meets king and queen' is the wordplay.
'irish republic' becomes 'roi' (abbreviation for Republic of Ireland).
'model meets king' becomes 'stere' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'roi'+'stere'+'r'='ROISTERER'
'in' acts as a link.
(Other definitions for roisterer that I've seen before include "Living it up" , "Noisy merrymaker" , "He's pushing the boat out" , "Noisy reveller" , "noisy party animal" .)