This is from Peadar Ua Laoghaire. Cad dó go raibh sé ag tagairt makes sense, but is it correct with an sgéil sin at the end?

I think that an scéil sin is added just to specifically show, what he was referencing to WITHIN/BY that story

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

Yes, you're right. It turns tagairt into a transitive verb, "referring or relating the story to something".