Green’s Dictionary of Slang

(Irish) a fool, a clown.

1689		Irish Hudibras in Bliss Irish Writings from the Age of Swift (1979) 60: And today make Holy-day / When all de Monaghans shall play / Ordain a statute to be drunk, / And burn tobacco free as spunk .
c.1735		Swift A Dialogue in Hibernian Stile in Bliss Irish Writings from the Age of Swift (1979) 30: None but garrawns, and I have seen him often riding on a sougawn. In short he is no better than a spawlpeen, a perfect Monaghan .