"Oidhche mhath, Uilleim."
Translation:Good night, William.
No a before a vowel in Scotland. (Note this difference if you speak Irish.)
If you are thinking that this name starts with a /w/ which is a consonant then it is certainly close to a consonant - there is not actually a clear-cut boundary between vowels and consonants - but it is more of a vowel in Gaelic then the English William. But however you pronounce it, it counts as a vowel.
As far as I know initial a, e, i, o, u count as vowels and everything else counts as a consonant. Unlike English where y and u are variable, and Welsh where w is.