Tannin is often found in bogs and swampy water. And tannin was a important part of the ‘tanning’ process for curing and preserving skins for usable leather.
Since the timeline is right for a culture that used a lot of skins for clothing and other paraphernalia, it’s possible that these Fiadhs were used for tanning skins.
Another point for my theory is that they were also built near—but not in—village sites. Tanning raw skins can produce some nasty smells that you wouldn’t want inside the living compounds.
Sounds more logical than giant soup pots or beer breweries.
Sounds more likely that the brewery idea, as appealing as it is.
Your hypothesis makes more sense than the idea of brewing the ale so far from settlements.