Are you sure? I just searched both sites and found nothing.
EDIT: Never mind, I found it… The search engine on both sites is not particularly good. Searching for “mac rí na héireann” on Litríocht brings up a lot of results and you have to hunt through them, and then when you finally do find it, the link doesn’t work. And on siopaleabhar, the title search brings up nothing, but if you hunt through the “new releases” section it is there. https://www.siopaleabhar.com/en/product/mac-ri-na-eireann/
And apparently, to answer the original query, this is actually a translation of the book by Colum and not an entirely different book altogether.
Thanks for bringing this to our attention!