At first sight, there's nothing particularly exciting about the Gaelic alphabet. (What do you mean the words 'alphabet' and 'exciting' shouldn't be used in one sentence? What do you mean I need to get out more?)
In its modern form, it consists of eighteen letters - five vowels: a, e, i, o, u and thirteen consonants: b, c, d, f, g, h, l, m, n, p, r, s, t. The only diacritical thrill is provided by the grave accent, which marks the length of vowels. In the past, the acute accent was also used in the case of e and o so that not only their length but also quality was shown in the orthography: è and ò were more open than é and ó. However, the spelling reformers of the 1970s put an end to that by replacing all acute accents with grave ones. Killjoys.
So, what's the exciting part, I hear you ask impatiently. Well, it's this rather neat tradition of associating each letter with a tree name. Its origins go back to the Medieval writing system known as ogham, although there's some debate as to how it exactly started. It's believed that when the characters in ogham were first named, only some of them were actually named after trees but because the letters themselves were called feda 'trees' or nin 'forking branches', they were all eventually reinterpreted as plant names and the so-called 'Celtic Tree Alphabet' was born.
Celts and trees, eh? You hear these two words in one sentence often enough, don't you?
But enough of this blethering, let's have them tree names. I list them as they appear in Dwelly's Dictionary not least because its recent editions sport a cover designed by Alasdair Gray, which, as you can guess, shows all the trees and shrubs representing the letters together with their Gaelic names. Here they are:
a - Ailm 'elm'
b – Beith 'birch'
c – Coll 'hazel'
d – Dair or darach 'oak'
e – Eadha 'aspen'
f – Fèarn 'alder'
g – Gort 'ivy'
h – Uath 'hawthorn' (no self-respecting Gaelic word begins with h; it's used to show aspiration and lenition)
i – Iogh 'yew'
l – Luis 'quicken tree'
m – Muin 'vine'
n – Nuin 'ash'
o – Onn 'gorse'
p – Beith Bhog 'soft Beith' (p is just a soft version of b; there was no p in ogham)
r – Ruis 'elder'
s – Suil 'willow'
t – Teine 'furze' or according to other sources 'holly', which seems to make more sense since 'furze' is another name for 'gorse' and we already have the latter associated with the letter o. Even Dwelly can let you down... (Thanks to Oolong for pointing out to me that gorse and furze are the same thing.)
u – Ur 'heath
Sin agaibh e ma-tha. There you have it then.
Sources:
http://en.wikipedia.org/wiki/Ogham
Dwelly, Edward The Illustrated Gaelic-English Dictionary 11th edition (Glasgow, 1994)