Green’s Dictionary of Slang

omadhaun n.

also ahmaudon, amadáin, amadaun, ohmadaun, omadaun, omadawn, omadhan, omadhawn, omadhon, omahdaun
[Irish amadán, a fool: a form of onmitán; from o?, a fool]

[mid-19C+] (Irish) a fool.