Girl, Irish one, small (4)
I believe the answer is:
iris
'girl' is the definition.
(girl's name)
'irish one small' is the wordplay.
'irish' becomes 'IR' (abbreviation for Irish).
'one' becomes 'i' (Roman numeral).
'small' becomes 's' (abbreviation - e.g. clothes size).
'ir'+'i'+'s'='IRIS'
(Other definitions for iris that I've seen before include "--- Murdoch, author" , "Bright flower or eye part" , "Flower - rainbow" , "who keeps pupil in?" , "Woman" .)