Could it be that the legendary Irish monk St. Brendan (appr. 489-580) came to Jan Mayen and saw Beerenberg on his epic skin boat (curragh) voyage in the north Atlantic? The story of St. BrendanÂ´s voyage is described in the latin manuscript Navigatio Sancti Brendani Abbatis and in chapter 23 he tells about how he found this island.
…full of slag and forges, without grass or trees. Brendan was worried, but the wind blew them straight toward it, and they heard the sound of bellows and thud of hammer and anvil. An islander came out of a forge, caught sight of the curragh, and went back indoors. Brendan told his men to row and sail as fast as they could to try to clear the place. But even as he spoke, the islander reappeared and hurled a great lump of slag at them. It flew two hundred yards over their heads, and where it fell, the sea boiled and smoke rose up as from a furnace. When the curragh had gone about a mile clear, more islanders rushed down to the shore, and began hurling lumps of slag at the monks. It looked as if the whole island was on fire. The sea boiled; the air was filled with howling; and even when they could no longer see the island, there was a great stench. Brendan said they had reached the edges of Hell.
Tim Severin, The Brendan Voyage
...or is he describing an island off the south coast of Iceland?