The calendar is modern, but the ogham letter behind each sign is not. Ogham is a genuine script, cut along the edge of standing stones in strokes across a line, and it survives in roughly four hundred inscriptions in Ireland, Wales, Scotland and the Isle of Man, most of them of the fourth to sixth centuries and most of them simply a personal name and a father's name. Twenty letters make up the alphabet proper, arranged in four groups of five called aicmi.
Nion is the fifth letter of the twenty, in the first aicme. Nin is not a tree word. It carries senses such as a fork or a loft, and the ash is a gloss the medieval commentators supply rather than a translation of the name.
Two things about the calendar follow from the alphabet, and both are visible in the list itself. The order is wrong: the attested sequence of the first group is Beith, Luis, Fern, Sail, Nin, but the tradition calls the alphabet Beithe Luis Nin, and modern scholarship reads that third element as a general word for a written character rather than as the letter N. Graves followed the name instead of the order, which is why Nion stands third here and Fearn fourth. And the count is a selection: thirteen months needed thirteen letters, so the first three aicmi were used with Cert and Straif dropped, and the fourth aicme, the five vowels, left out altogether. Neither the reordering nor the pruning is recorded in any medieval source. The letter is old. The month is not.