I am in no way an expert in gaelic music, in fact there is more that I do not know than I know. I recommed to read Craig Cockburn's Article on the subject for everyone who wants detailed information.
This is just meant as an overview for those who were previously unaware of the fascinating and rich song tradition of the gaelic speaking people. I have a short comment on the song sites themselves for each category.
Also please note that this is full of my personal opinion and should not be taken as gospel in any way *g*.
Gaelic is a so-called "minority language" that is spoken by less than 100 000 people nowadays.2001 there was a new census, but I believe the 1991 one resulted in about 60 000 speakers. The main region where Gaelic is spoken as an everyday language is the Western Isles, or Outer Hebrides on the West Coast of Scotland. There is however a strong Revival or Renaissance phenomenon, that has lead to there being a great number of learners. The Gaelic Renaissance is a huge field of research and can not be covered here.
Scottish Gaelic is part of the same language family as Irish Gaelic and Manx Gaelic (Isle of Man). The other branch of celtic languages includes Welsh, Breton and the now only artificially revived Cornish. They are both part of the indo-european language family. The main characteristic that people notice about gaelic is it's "strange" phonetics. It works with a very different system than most of the familiar romanic, germanic and slavic languages in Europe, which often leads to a horrified awe on the side of learners. *g* The language also contains many sounds that are not part of the english language. I found that having german as a native language helped me with many of the sounds. Ä, ö and ü, as well as the various guttural sound in german are very similar to gaelic sounds.
Gaelic songs can be roughly divided into three categories: work songs, puirt-a-beul (mouth music) and free-style songs. Waulking songs are the prevalent kind of working songs. Their main characteristic is their strong rhythm and the use of usually short verses that alternate with a chorus that is partly made up with so called vocables, like Faill ill o ro, ho ro eile, ho i og i o, etc. In this kind of song the rhythm takes precedence. The same applies to the puirt-a-beul, which are short dance tunes with a light-hearted and often nonsense text. They sound very alien to the ear unused to them, but they have an immense drive and are a lot of fun to sing. They work in most cases with repetitions of very few lines and are sung in sets, like in instrumental music.Waulking songs and puirt are sung unaccompanied, but in modern gaelic music peformances, they are sometimes arranged with instruments, which, if carefully done, can add an attraactive note to this form of songs. My personal taste, however, leans toward singing these song unaccompanied. Especially waulking songs do unfold their power best only with the monotonous stomping sound of the cloth beating on table.
The other form of songs, which I called free-style above are very often love songs, songs of praise of a person, a place or a political comment concerning the gaelic language and culture. In these songs the gaelic takes precedence, meaning that it is important to pronounce the words in their proper rhythm, with the correct stresses on the syllables. It is emphasised by some that the words are more important than the music, that the aesthetic and skilled use of the gaelic stands in the foreground.
A few of these songs are of a slightly rhythmic nature as well, but usually the singer has some freedom in his interpretation. It is therefore a bit hard to write down these songs. In gaelic music one has to hear a song to get the feeling, as is the case with so much music all over the world.
It is sometimes suggested that there is a typical "traditional" style of singing these gaelic songs, but I must admit that I believe it to be a mistake to press this music in a form, that might be just one way of doing it. One sometimes hears a certain nasal style of singing that is considered very traditional by some.
But one can surely say that one characteristic of gaelic song is that it is often sung by "natural voices", meaning people who use no classical technique. I have sometimes heard songs performed by people who sang in an almost operatic style, and that was in my opinion, honestly, plain awful.
My suggestion is to get recordings by the following artists to get an impression over a number of styles and voices.
If you like these you will probably already be addicted as I am ;-)) Without wanting to romanticise the gaelic music, it has something very compelling to it.