Icelandic's other unique letter, the thorn, is obviously Runic (and near the front of the Futhark). Eth was not defined in the "First Grammatical Treatise" of 1140-1180. It seems like both the Runic and the Latin orthographies for Norse were considered complete without a symbol for its sound. Wikipedia says the eth "originated in Irish writing" but does not explain how it might have been transmitted into or out of Anglo-Saxon, nor how Icelandic came to use it.
Where did the eth come from, and how and when was it chosen for use in Icelandic?