twins (one pair) = cúpla
twins (more than one pair) = cúplaí

If I am referring to one pair of twins, is it na cúplaí because there are two individuals? As I am writing this, there actually doesn't seem to be an option to use 'an' instead of 'na'. Have I just answered my own question and if I have, am I correct?

(Edited to include Lughaidh's info below)

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Cúpla comes from (Norman) French "couple" which means "group of 2 persons". So: twins = cúpla. If you say "cúplaí", then you're talking about several "pairs" of twins

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

O.k. so now that we have established that I had that completely bun os cionn, does that mean one pair of twins is actually an cúpla? And if that is correct, how would you refer to one individual twin ie one person? Or is it that 'an cúpla' refers to one individual and 'na cúpla' refers to one pair of twins and 'na cúplaí' refers to more than one pair of twins? I wouldn't mind, but I don't actually know very many twins and I don't really go around talking about them anyway and, after this, I don't think I ever will......

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An cúpla = the pair of twins
Na cúplaí = the pairs of twins

one twin = leathchúpla ie. a half-pair of twins
But I don't think you often need to say "one twin"...

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

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I have twins. You're right, it's not usual for people to talk about one twin - but it does happen! For example, meeting new people and talking about your chldren:
'I have twins. They are identical but one twin wears her hair long and the other short" etc etc.
I was just struck by 'leathchupla' not being a very acceptable description of a person! Goes against all tenets of child psychology!

Surely, from a psychological damage point of view, “one twin” is an equally unacceptable description of a person. I see no difference between “leathchúpla” and “one twin” other than semantics. Either way, the frame of reference is one half of a unit.

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Táim ag foghlaim Gaelainn na Mumhan

Tá fáilte roim nach aon cheartú!
I am a learner. Any translations offered are practice and should not be used unless confirmed.

Works the same with singlets. I have 2 sons. One son is tall, the other is not.

It was the 'half' bit that struck me.

It's also how you say "one eye", "one foot", e.t.c.

Leathshúil = one eye.

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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