That’s the Gaelic name for the tune called “Sleep, Dearie, Sleep” in English. It was used instead of “Taps” or other tunes in some of the Scottish regiments, which is why it’s now used for funerals, like that of Queen Elizabeth II.
The “official” English lyrics are by Sir Walter Scott. Allegedly. I have not found them.
There is a very old tune called “Caidil Thus’ A Ghaoil” which is allegedly a tune learned from the fairies, from overhearing a fairy mother singing a lullaby. (Which usually means it “sounds weird.”) It looks like this is the first part of the current pipetune, and that somebody (human) composed a second part to spin it out longer. (And hoboy, the lyrics turn out to sound pretty human….)
Caidil thus’ a ghaoill (3X) ‘N leaba chaol nan clair.
Cha bhi mise bhuat (3X) Mach air uair na dha.
Caidil thu o! (3X) Dean o! Tamh!
Sleep, thou, o beloved (3X), in your narrow bed of boards.
I will not be away from thee (3x) When out about, it’s the two [of us].
Sleep, thou, o! (3X) Do it, o! Rest!
Another set of Gaelic lyrics, which seems to cover the longer modern tune:
Caidil mo ghaol (3X) Caidil, o, caidil, mo ghaol, mo ghaol.
Tha’n uile math, Na gabh dragh, Caidil o caidil, mo ghaol.
Sleep, my beloved (3X) Sleep, o sleep, my beloved, my beloved.
Everything is well, Don’t worry, Sleep o sleep, my beloved.
Sheet music from a book of regimental songs and bugle calls, with more information. The Seaforth Highlanders and the Cameron Highlanders both used the song to signal “Lights Out.”
“Sodger, lie down on yer wee pickle straw.
It’s no’ very broad and it’s no’ very braw.
But sodger, it’s better than naething at a’,
Sae sleep, sodger, sleep.”
More old sheet music of the tune.
The military lyrics are very similar in spirit to the later song “The 51st Highland Division’s Farewell to SIcily” by Hamish Henderson (ttto the pipe tune “Farewell tae the Creeks” by the piper James Robertson of Banff), which was filked as the Dorsai song “Green Hills of Harmony.” (Listen to that YT video, and notice that the original song actually had an _extra part_ that has been worn away by the folk process. It’s pretty, but yeah, I can see why it left us!)