In Old Irish, the letter name was Beithe, which is related to Welsh bedw(en), Breton bezv(enn), and Latin betula.
[1] The Auraicept na n-Éces contains the tale of the mythological origins of Beith[2] This moreover is the first thing that was written by Ogham, [illustration of seven b's, in Ogham script] i.e. (the birch) b was written, and to convey a warning to Lug son of Ethliu it was written respecting his wife lest she should be carried away from him into faeryland, to wit, seven b’s in one switch of birch: Thy wife will be seven times carried away from thee into faeryland or into another country, unless birch guard her.
It replaced Ifín ᚘ, one of the "original" five Forfeda likely named initially pín (influenced by Latin pinus) with an original value [p] but whose phonetic value was altered to a vowel diphthong due to later medieval schematicism.
[4] While medieval and modern neopagan arboreal glosses (i.e. tree names) for the Ogham have been widely popularised (even for fade whose names do not translate as trees), the Old Irish In Lebor Ogaim (the Ogam Tract) also lists many other word values classified by type (e.g. birds, occupations, companies) for each fid.
fili) or poets of this period learned around one hundred and fifty variants of Ogham during their training, including these word-list forms.