Does anyone recognise bolacán on p155 of Cnósach Focal ó Bhaile Bhúirne: gread-thine do bholacán agus do bhainne caereach. I think this is related to bolcán in Dinneen - strong spirits made from black oats and used by the poor. What are these spirits called in English?

If it's a spirit made from oats you're looking at some variety of whiskey or poitín.

In the modern day there are strict restrictions on what constitutes "whiskey". At a legal level, there are government regulations regarding what can be called "Irish Whiskey", "Scotch" or "Bourbon". There are also a lot of general expectations about what how a whiskey, or variety of whiskey should be made and what it should be made from, even if they aren't enforced in any legal sense. Historically, though, "whiskey" was a bit of a catch-all term for any spirit made ultimately from grains traditionally grown in Ireland. This would have included barley and oats, etc. but obviously not the likes of rice, and other grains cultivated in Asia.

As for whether there's a more specific term than "whiskey", I think poitín probably covers it. Rightly or wrongly it gives me the sense of a somewhat less regulated distillation process, possibly without any maturing, and likely something that individuals who could not afford to buy regulated whiskeys might make for themselves. If this interpretation of the term is acceptable, then it seems the most appropriate candidate for describing any variety of "strong spirits made from black oats and used by the poor".

It sound a bit like moonshine!

Well yes, more or less the same concept, though I think moonshine is typically made from corn meal rather than other types of grain like poitín is. I also associate moonshine with America more so than Ireland, though I suspect it's a development of the Irish/Scottish illicit distilling tradition using locally available crops and resources.

It's odd that Dinneen has bolcán and CFBB has bolacán. There wouldn't normally be an epenthetic vowel between L and C. The only possibility is that this is bolgthán, or might have been (correctly or otherwise) reinterpreted as bolgthán, thus requiring an epenthetic vowel and yielding the form in CFBB?

I would have suspected it was the Munster epenthetic vowel at play, and expressed in the orthography in CFBB, but that if a Munster speaker were to pronounce bolcán it would be the same as bolacán. I didn't think too much about it, but as you say, it is unexpected between L and C.

I'm now told this means: a strong fire is required to distil rotgut or prepare sheep's milk.

I've now been told that CFBB incorrectly spelt this bholacán whereas it should be mholacán. See under mulchán in Dineen's dictionary where the exact phrase is glossed as "use a strong fire" when making dried baked curds.