Northern Ireland company aboard vessel finding mythical beast (7)
I believe the answer is:
unicorn
'mythical beast' is the definition.
(I've seen this before)
'northern ireland company aboard vessel' is the wordplay.
'northern ireland' becomes 'ni'.
'company' becomes 'co' (abbreviation).
'aboard' indicates putting letters inside.
'vessel' becomes 'urn' (urn is a kind of vessel).
'ni'+'co'='nico'
'nico' inserted inside 'urn' is 'UNICORN'.
'finding' acts as a link.
(Other definitions for unicorn that I've seen before include "Fabulous animal like horse with horn" , "Fabulous horned animal" , "Imaginary, horned horse-like creature" , "Animal rumoured" , "fabulous thing to behold" .)