I want to say: in An Soísgéal as Leabhar an Aifrinn. How do you combine in and an? Is it "in An Soísgéal as Leabhar an Aifrinn"? The in+an thing is a bit odd.

I think it's

in An Soísgéal as Leabhar an Aifrinn

in is used (even in the old spelling, where i n- might be expected) is used before titles etc. even when eclipsis could follow:

in Fiche Bliain Ag Fás

Though sa might be acceptable. My guess would be that in would be used if there is a pause between it and the title, where sa (if possible) would be used in flowing/rapid speech.

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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

Thank you.