Labials or B P F M
This is isn't half as bad as what you have to learn about liquids. Trust me.
The word labial ultimately comes from the Latin word labium meaning lip and surprise surprise, describes sounds which involve the lips. Gaelic just has four of these ... well, kind of ... there's broad and slender ... and lenition. Ok ok, maybe it's a bit more complicated thatn I made it sound at first. One thing at a time though.
Common Gaelic had 8 labials, plus their lenited fricatives: [b] [bʲ] [p] [pʲ] [f] [fʲ] [m] [mʲ] [v] [vʲ]. In modern Scottish Gaelic this has changed to [b̊] [b̊ʲ] [p] [pj] [f] [fj] [m] [mj] [v] [vj].
B & P
The main difference between [b] and [b̊] is one of voice which we explained here earlier and the difference between [b̊] and [p] is one of aspiration which is explained on the same page. But to some it up again:
So what's the [bʲ] [b̊j] thing about?
Well, in Common Gaelic [bʲ] was a voiced palatalised labial - kind of a b with pursed lips - which it still is in Irish. In Gaelic though this slender [bʲ] has been lost though except in certain circumstances where the [b̊j] sound is followed by a [j] glide. This tends happens when you get a formerly slender [b̊] which is followed by a back vowel. Back vowel?
The vowels we make can be categorised according to where they are made in the mouth. let's look at the following diagram:
If you start with [i] and then go down to [e] [ɛ] [a] and then over to [ɔ] [o] and [u] in one smooth transition, you will notice how the place of articulation moves way back in your mouth. That is why [ɔ] [o] and [u] are called back vowels. The [aː] vowel in Gaelic groups with the back vowels as well. Don't ask why (well, ok ... because as you move downwards in the diagram, your tongue not only moves back but is also lowered and the distance between the two low vowels [a] and [ɑ] is so relatively small that languages which have only one a sound often group [a] with wither front or back vowels).
To recapitulate, the back vowels in Gaelic are: [a] [ɔ] [o] [ɤ] [u] [ɯ]. This also includes [au].
Sooo ... let's make this clearer with some examples:
There is one tricky issue: with vowels spelt ea, you only get the glide if the following consonant is 'strong' (for the want of a better term): [ɫ̪] [n̴̪] [r̴] [ɹʃt̪]
Eu [eː] incidentally doesn't count - because it's not so much the spelling that governs this phenomenon but the pronunciation and eu is simply a long vowel. When broken into [ia] you do get a diphthong, but short [a] is not considered back so there is no glide in words like beul [b̊iaɫ̪].
The same rules apply to the p's so we won't give them extra coverage here.
F & M
There's not much to say about these sounds as they are pronounced exactly as they are in English. Again, the rules stated for glides about apply here too:
As do the exceptions mentioned above, so all in all quite a regular bunch.
B P F M
If you had a glide in the unlenited word, you get a glide in the lenited word. Not too tricky. The only difficult bit to watch out for is f as it lenites to zero ie no sound at all. This is tricky for two reasons: first because you have to get used to a sound which leaves not trace at all when lenited (unlike all other sounds which leave some sound behind when they lenite) and because there are two different outcomes depending on whether the lenition is caused by the definite article or something else in f clusters.
Normal F Lenition
But with the definite article, strange things happen. Well, not so strange but it's a pain having to watch out for something else. An [fɾ] or [fl] after the definite article (when it causes lenition) strengthens to [r̴] and [ʎ] respectively. Oh, and the [n] of the article strenghtens to [n̴̪] and [ɲ] respectively:
This strenghtening is something that happens quite regularly to the definite article. The other sound [fɫ̪] isn't affected because [fɫ] doesn't exisit anymore (well, unless you're from Harris, in which case you have [fɫ] which can strengthen to [fɫ̪]). If you speak a dialect which happens to allow slender [ɾʲ] in an initial cluster ie [fɾʲ], then you strengthen that to [r̴] too because initial slender and broad r are exactly the same. Quite logical all that really!!