I was told in Kerry that the gender of a word which is a diminutive in ín is typically the same as the basic word.

Diarmuid Ó Sé seems to confirm this on p.483-484 of his book Gaeilge Chorca Dhuibhne in points 864-866. He gives:
Bóín bheag, Abhainnín bheag
and
Is ionann inscne don fhoirm dhíspeagtha agus don bhunfhoirm de ghnáth.

In fact he gives the interesting Crúibín, which although masculine, unlike Crúb, gives Crúibín mhuice.

He also lists Réilthín as an -ín terminating word without a base word, there are four other words like this given, all of which are masculine.

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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

A Loin Duibh, you seem to have identified the solution!

Réiltín is the ordinary word for "star" in the great southern province of Munster - and no longer seem as a diminutive of anything - and so that means it can just be masculine - it doesn't have to be seen as a form of réalt.

But it's still right to refer to it as í, uirthi, aici etc - just as bád, cailín and some others take "the feminine of reference".

-óg / - eog is also a diminutive ending and it makes words feminine

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

On that topic, are -óg or -án still used productively as a diminutive anymore?

For example, if we had leabhar or some other noun, every native speaker I've met produces leabhairín or the equivalent for the dimunitive. I've never heard something like *leabharóg or *leabharán. I know that -án was originally an insulting dimunitive.

Also, if anybody knows, if cailín is girl as we all know, but what originally was a Cail?
Or for a fuinneog a fuinn?

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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

According to the original Teach Yourself Irish, my bible of Munster Irish, it says that the masculine suffix -án "is no longer productive, and is sometimes pejorative."

Then it goes on to say: "The feminine suffix is -óg. It is not used to make new diminutives. All words ending in -óg belong to the second declension …"

* edit:
According to "The Wee Falorie Man's Crackpot Theory of Word Origins" : I think the word cailín might come from the same word that gearrachaile comes from – maybe the original word was something like "caile". Interestingly, the word gearrachaile is also masculine.

I think you could add cailleach to that. Cailleach is feminine though.

It seems that all Irish women are transgender (they were masculine until they grew up and became mná).

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Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

It's from caile, an old word meaning "maid".

cailín> caille > pallium (Latin) meaning veil.

The loss of initial "p" suggest that pallium was assimilated into the Irish language before 550 AD. The loss of um termination corresponds to the usual apocape rules.

In essence caille means the veiled one. Cailín meaning the little veiled one. There is much debate surrounding the word in the Old Irish poem Caillech Béirri, of which Ó Crualaoich has written extensively on.

See eDil under caille and caillech: http://edil.qub.ac.uk/dictionary/result ... 2&limit=20

Fuinneog (fuinneóg) > fuindeóg is derived from Old Norse vindauga meaning window.

See eDil under fuindeóg: http://edil.qub.ac.uk/dictionary/advres ... &option3a=

A more common word for window in Old Irish is senester and later fenester from Latin fenestra.

See eDil under seinistir and fenister: http://edil.qub.ac.uk/dictionary/advres ... &option3a=

The switching of initial Latin "f" for "s" in old Irish was a common trait e.g. Latin furnus > Irish sorn.

The modern Welsh cognates are (f)fenest and (f)fenestr

See: http://www.geiriadur.net/index.php?page ... part=exact

Cian.

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Please wait for corrections/ more input from other forum members before acting on advice

We use "ín" as a diminutive.

I wonder if "-óg or -án " is a Munster thing?

I've always said "realtóg" for "star".
Realtógaí (plural) and Realtóigín (dem)

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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-án isn't really productive (except in An Coiste Téarmaíochta )
-óg/-eog end loads of words

Fuinneog isn't a good example because it's not the diminutive -eog ending ; as you said, it comes from the -auga word (old norse for "eye", vindauga = the wind-eye).

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