Well, most of the time you say "súil".
You'd rather use "tá sé ar leathshúil" or "tá sé ar leathchois" to say the person has only (good) one eye and one (good) leg.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Really! Does that mean that súil amháin in incorrect or just an alternative way to say it? And how does that link into the surname Ó Suilleabháin or is it a myth that that is to do with one eye.

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As Lughaidh said, it's not really "one eye" in the usual sense, but it is used in a lot of idioms:

Do chaithinn leathshúil ar tigh Shéamais gach maidean nuair a bhíos ag bun an bhothair = I'd toss a glance at Séamas' house every morning when I was at the bottom of the road.

Leathbhróg is one shoe (of a pair).

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal