Does anyone recognise this? Tá móirín 'na suí - the sun has set. This is from Cnósach Focal. Does it derive from an affectionate term for the sun, being a "big thing"?

I know the word, but not the origin. I've heard something like that sentence once or twice. Móirín is kind of a funny word to me, sounds like "little big thing."

I've only ever heard Móirín used as a female first name. It's an adaptation of the female name Mór with the diminutive suffix used to denote "dear" or "little". I'm going to suggest that this is most likely what you have here as well, as nouns ending in the diminutive suffix -ín are typically masculine, and so you would expect 'na shuí. I suspect that the expected lenition is being suppressed here by the understanding that this is actually a reference to a woman with the name Móirín, and so móirín likely can't be taken as a reference to the sun in isolation, only as a part of the wider idiom used here.

I think the gender of diminutives an area of language requiring a monograph! Réilthín is masculine, but cathairín is feminine. I think this is because cathairín is felt to be the diminutive of a feminine noun, cathair, but réilthín is no longer seen as a diminutive of réalt, but as the actual word, the real word for "star" in Muskerry Irish. I'm sure there is plenty more to say on this subject....

Certainly, there are plenty of exceptions. But, in combination with the fact that Móirín is a personal name, I'm still inclined to think the interpretation "the sun has set" is idiomatic rather than direct, and that móirín can't be interpreted as "sun" in isolation.

With that being said, I'd be happy enough to change my mind if somebody has an example of móirín being used as a direct translation of "sun". I just couldn't find any myself.

my grandparents had this word, and never used it for anything but sun, but maybe its an idiom

Well that changes things then. It must have been widespread, at least across Munster in that case.

I wonder what its relationship to the name Móirín is, then.

Yes, also found as Móróg or just Mór. It's a woman's name, often Anglicised to Martha, and is used as the personal name for the Sun.

"Móróg dhuit" being said on a sunny day with "Móróg, Pádraig is Muire dhuit" in response.

Britannia, the national personification I mean, was Mór Bhuí in Irish.

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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

https://www.momswhothink.com/baby-name/maureen/

Yes, I know. An "amateur" site. Do you think this explanation is plausible?

"Maureen is the anglicized version of the name Máirín. This name is the Irish diminutive of Marie adapted from the Latin epithet stella maris (meaning “star of the sea”). The Old Irish symbol of Mary is the rosa mystica emblem."