There’s often "they" in English nowadays as a gender neutral pronoun.
And sometimes I see "iad" in Irish used in the same way.
E.g. "aon duine a bhfuil … acu" instead of "… aige" or "… aige/aici"

But unlike "they", iad/acu never had been anything else but a plural pronoun in Irish, hadn’t it?

Well, there are some features of a Sprachbund in the British Isles, despite the fact that Irish is so different from English otherwise.

I'm not sure that "they" as a supposed gender-neutral pronoun has a long history in either language. But "they" as a pronoun referring to a collective singular is in both languages:

1. English: the majority ARE in favour; the government ARE introducing a law; the married couple ARE kissing in the street. (Some of these are optional in English and some aren't. You can say "the government IS", but in British English you can't say "the couple IS").
2. Irish: "That is the congregation": Sin iad an pobul, not siné. (This sentence is quoted from Peadar Ua Laoghaire's Notes on Irish Words and Usages). Táid muinntir na h-Éireann bodhar agus tá clann na h-Éireann balbh! (Also from PUL.) isiad muíntir na h-Éirean na daoine is macánta sa domhan. isiad clann Dé iad.

But "aon duine" is just not a collective noun in that sense. Are you sure you're quoting native Irish and not the Irish of L2 speakers?

In Irish text corpora, I could find only a few examples.

Aon duine a bhfuil aithne acu ar Thráigh an Choma tá a fhios aige gur minic a phléascann tonnta ollmhóra anuas ar an dtrá agus nach aon áit í atá oiriúnach do naomhóg bheag éadrom.
(Mícheál de Mórdha from Dún Chaoin, An Rialtas ab Fhearr, 1993) Strange enough, he uses acu and aige in the same sentence

[...] is iongantach go dtig le fear a ghabhail 'un donais agus náire agus buaidhreadh a thabhughadh do 'ach aon duine a bhfuil cion acú orthú.
(Domhnall 'ac Grianna: Gadaidheacht le Láimh Láidir)

Some results with duine are obviously plural though in singular form, e.g. "Is beag duine a ..."
So I don't mention them in detail.

And there are some examples with corr- as prefix (apparently also felt as plural):
Má tá sé cosamhail le corr-dhuine a bhfuil aithne agam ortha
(Séamus 'ac Grianna, Saoghal Corrach)
Tá corrshéiplíneach a ngortaíonn an imirce iad
(C. Ó Ceallaigh, Sclábhaíocht, 1990)

Well, it must be possible, then. And it makes sense, from a Sprachbund point of view.

A bit random side note, but this reminds me of a sentence in Séadna that surprised me when I first read it – although no pronouns here:



With a grammatically singular noun subject referring to a group as a subject to explicitly plural verbs. Years of learning Munster Irish later it feels very natural to me, but then I was really surprised to see a plural (“synthetic”!) verb followed by singular noun subject.

Yes, Silmeth, this is natural and correct Munster Irish - and yet English has the same thing (the couple ARE kissing in the street), although maybe not in US English (where the language is under heavy L2 influence from people who were originally German-speaking). This supports my view there is a British Isles Sprachbund.