Fire Fern is a tall plant with long spear-like leaves that look like those of grass. The fire fern appears to have a deep red color overlaying green. Dagur says the fire fern grows under Elm Trees. This may indicate some sort of relationship (symbiotic or parasitic) with the trees, for instance, true ferns often grow under trees.
The fire fern also seems to have a defense mechanism, like Nettles do. Touching them with bare hands will scorch the skin, as if on fire, as we see happen to Hiccup in Enemy of My Enemy.
In actuality, there is a type of Oxalis called "Fire Fern" (Oxalis hedysaroides) due to its red color. However it is a small plant known for its "shamrock" shaped leaves and is native to the Caribbean. Incidentally, other members of the Oxalis Genus do occur in Scandinavia and were sometimes used for food historically.
In the How to Train Your Dragon Franchise, fire fern is part of an herbal concoction for Dragons to combat the effects of Dragon Root. Other components include: fresh water, Bitterweed, and Purple Oleander. The ingredients are heated together to make a tea. The final product is purple in color and is fed directly to the affected dragon. In "Enemy of My Enemy", Toothless was hit by an arrow tipped with Dragon Root. It is not clear if this antidote is useful only on Dragon Root entering the blood stream or also for other means of delivery, such as aerosolized Dragon Root. It is also not discussed if fire fern has use in human herbal medicine.
Fire fern is only seen in one episode, "Enemy of My Enemy". Toothless is shot with a Dragon Root arrow by Dragon Hunters who set a trap for Hiccup and Toothless. The two barely make it to a nearby Unnamed Island. The Dragon Hunters pursue. Hiccup attempts to save himself and hide Toothless, who is extremely lethargic, but not unconscious from the Dragon Root. A mysterious man who is later revealed to be Dagur tried to help Hiccup. Because of the past, Hiccup has a hard time trusting him. Dagur recalls seeing Ryker Grimborn making an antidote for Dragon Root, and the two humans set out to find the ingredients.