A while back I posted a thread, and was corrected for saying cé chomh deocair, which isn't used in Munster Irish (instead: a dheocracht/cá dheocracht/an deocracht is...). Recently I read in an article on Tuairisc.ie, written by who I assume (based on other vocabulary and grammar, etc.) was an Ulster speaker cad é chomh costasach is.... Would this also be a fair construction in Southern Irish, of course, within clauses (e.g. is deocair a dhéanamh amach cad é chomh costasach is atá praghas oibre i dtíortha eile lasmuigh d'Éirinn? Many thanks in advance!

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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

It is at least to be found in a novel by Mícheál Ó Súilleabháin from Imileá, Baile an Fheirtéirigh,
"Peacaí na nAthaireacha" (https://www.litriocht.com/t%C3%A1irge/p ... haireacha/):

Ní thuigeann an strainséir, a Bhreandáin, cad é chomh domhain agus bhí a bpréamhacha sáite ag an Slua Sí i gcultúr na ndaoine anseo ná cad é chomh deacair a bhí sé iad a stothadh.

Ach cad é chomh dona agus tá sé?

Go raibh míle maith agat. According to ainm.ie Ó Súilleabháin was born in the year 1917, I wonder if there is a previous version of this book or if the standard one is the only edition released. From the text that you provided, there is a lot of standardisation of what would have been the author's original speech, for example, doimhin is said instead of, comh instead of chomh (for the most part), etc. etc.

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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