A letter in ogham is a family and a number of strokes. Nothing about the shape of an individual character has to be learned separately.
How the letter is formed
Nion is the fifth and last letter of the first family, Aicme Beithe, and is cut as five strokes on that family's side of the line. Five is the maximum in every family, and the letter that carries it closes its group.
Reading a five
Five parallel strokes are the hardest character in the system to read on a damaged stone, because a lost or a doubled stroke turns the letter into a different one rather than into nonsense. There is no redundancy anywhere in ogham: every count from one to five is a valid letter in every family, so no miscount is self-correcting. Epigraphers reading worn inscriptions treat five-stroke characters with particular caution for exactly this reason.
The sound
Nion carries the value N. It is common in the surviving corpus, which consists almost entirely of personal names in the genitive case cut on standing stones in Ireland and in Irish-settled parts of Britain, dated on linguistic grounds to roughly the fourth to the sixth centuries.
Two traditions, kept apart
The ash tree attached to this letter comes from the manuscript glosses, several centuries later than the stones. The calendar arrangements that circulate today descend from a twentieth-century reworking, later still. This atlas records the epigraphic form, the manuscript gloss and the modern scheme as three separate things.