It sounds like an ost house and place to kiln the malt.
To malt barley you put it on a concrete or stone floor. You soak the barley and allow it to germinate overnight. In the presence of water the hull breaks down, more water enters, the germ begins to sprout, and enzymes are produced that start to convert starch to sugar. During the process heat is produced and you rake the pile around to encourage even germination.
You do not want this to go on too long. At some point you need to kiln the sprouting barley to stop the germination. We do this on large metal trays heated underneath. Since metal was not common In ancient Ireland, I would not be surprised if they heated rocks and placed them on the ost house floor with the malting barley. You need to heat your barley until your malt is dried and/r carmelized for a darker ale. When this is done you can store it for longer periods until you are ready to make your beer.
I once malted my own barley one time on a tray in the kitchen oven. Someone had given me half a bag of raw barley. The beer was a bit on the skunky side, but it had a kick. (It was the Belgian yeast.) I don’t drink so much now, but I know that when the time comes there will be room in someone’s compound for someone who knows a bit about brewing!
:’)