On Irish author said to be merry (7)
I believe the answer is:
rejoice
'be merry' is the definition.
(to rejoice is to celebrate)
'on irish author said' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'irish author' becomes 'Joyce' (novelist James Joyce).
'said' shows a homophone (sound like).
'joyce' is a homophone of 'joice'.
're'+'joice'='REJOICE'
'to' is the link.
(Other definitions for rejoice that I've seen before include "Show great delight" , "Exult -- make merry" , "Show great elation" , "Be exultant and glad" , "Express great happiness" .)