G'day,
Now my candle is burned out (or Noo my cannle is brunt oot) is probably meant to be taken literally. Snotter: small lamp. So the second line complements the first.

MacColl in "Folk Songs and Ballads of Scotland" says: It was the custom in N.E. Scotland for ploughmen to be hired at seasonal hiring fairs. During the period of service, the ploughmen slept in small buildings (bothies) set apart from the farmhouse proper. When the day's work was done and the evening meal ended, they would often amuse themselves by singing and making up songs. 'The Barnyards' ia a perfect example of the bothy song.

This probably helps in understanding the song.

Cheers,
Alan