This character, simply called Iron Mason in the "Sandbusted" Credits, appears briefly in that particular episode.
Selling His Wares
The Iron Mason has a stall selling mostly weaponry at the Northern Markets. He has many items including maces, swords, and spears. Astrid visits his stall along with the twins and Fishlegs, attempting to find a betrothal gift to get for Hiccup. The Iron Mason tries to showcase his weapons for Astrid, but upon hearing that the gift recipient is not really into weaponry, he tries some other items such as a "decorative plate". He also has a map to the "Treasure of Tears". Astrid bypasses all of these things. However, he is later able to sell the treasure map to the Twins.
The Iron Mason has a dark brown beard split in the middle and tied into two pony-tails of a sort. He has a standard Viking helmet with two upward curving horns. He wears a wine colored tunic and a leather kilt with leggings. He also wears bracers on both arms.
The Iron Mason appears in the episode "Sandbusted".