Ironwood Fruit is a rare ingredient that can only be harvested from Iron Tusk Cave on the island of Giant's Tooth. This island can only be reached if you choose the Altar of Nothing during the quest A Dying Wish. You will be sent to the island to retrieve a single Ironwood Fruit, which will be marked as a quest item. However, the ingredient will no longer be marked as a quest item after the quest has completed, and can be harvested and used freely.
Six Ironwood Fruit plants exist, namely the rare Ironwood trees that grow within Iron Tusk Cave. The fruits on these trees respawn.
A single cooking recipe calls for the use of Ironwood Fruit. Ironwood Soup can be made with one sample of Ironwood Fruit, one Charred Skeever Hide, and one Salt Pile. You may need to cook a bowl of this soup during the quest A Dying Wish in order to treat ataxia. In fact, drinking this soup will cure all diseases.