Nion is a letter of ogham, the earliest form of writing in the Irish language, cut as sets of strokes across a central line. It stands for the sound N and belongs to the first (Aicme Beithe) of the four aicme, the groups of five into which the twenty principal letters are divided.
Letter, tree and later meaning
Ogham is a genuine antiquity: some four hundred inscriptions survive, most on standing stones in Ireland and western Britain, cut between roughly the fourth and sixth centuries. Each letter also bears a name, and medieval Irish scholars glossed many of those names with trees and with word-kennings; Nion is linked to the ash, from which its letter-name derives.
The divinatory use of ogham, here the sense of connection and the world-tree, is not attested in the early inscriptions. The reading of ogham as an oracle, and much of the elaborate tree-lore attached to it, belongs to the medieval glossary tradition and, above all, to the modern neo-druidic revival of the twentieth century. It is presented here as that later tradition, laid over a script whose first purpose was to record names in stone.