Aicme Muine is the third group, named for muin, and its five letters are muin, gort, ngetal, straif and ruis, giving the sounds m, g, ng, z or st, and r. In the script these are cut as one to five strokes crossing the stemline diagonally. Two of the five, ngetal and straif, represent sounds whose value is uncertain and which are rare or absent in the inscriptions, and the letter names themselves are in several cases obscure; the meanings given to them in the medieval Irish tradition are partly guesses by later scholars working on a script that had already gone out of use. This is worth stating because a great deal of modern writing treats the ogham letter meanings as a settled body of tree lore, and they are not.
The name muin is at least as unsettled as uath: Old Irish has three unrelated words spelled muin, neck or upper back, related to Welsh mwn and Latin monile, a neck ornament; wile or ruse; and love or esteem, and which of the three, if any, actually named the letter is not known with confidence. The Auraicept's arboreal tradition instead glosses the letter as vine, a reading modern scholarship treats with particular suspicion, since the ordinary Old Irish word for a grapevine was finemain, a loan from Latin, and grapevines are not native to Ireland, making the vine gloss look like a later, retrospective tree assignment rather than a genuine original meaning.
Sources A Guide to OgamDamian McManus with Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.)