Puirt a beul (Gaelic for "mouth music" and pronounced phonetically as "poorssht ah buhl", slurring the s sound and rhyming beul with "gull", not "tool") is a Scotch-Irish singing form in which the singer sings a cappella in a style reminiscent of scat, though with a folkier, slightly less punchy approach. A lot of scholars believe that the instrumental versions of these songs existed first, and the puirt a beul evolved as a kind of mnemonic for musicians to remember and pass on to other musicians.
The music itself is more interested in its syncopation and rhythm of the tune than in creating melodies, and almost no harmonies exist as most puirt a beul is the work of a solo vocalist (although there a number of accompaniments which use 2, 3, and 4 vocalists.)
Generally the puirt follow traditional dance tunes, featuring a slower and more descriptive verse with a lively, easy to follow chorus. The lyrics are almost always bawdy, often using the syncopation and rhythms of the music to sell a pun or linger on a dirty rhyme. (Of course, they're all sung in Gaelic, so good luck trying to figure out the puns in that language.)
It was probably made most famous by the Scottish band Mouth Music, which combined puirt a beul with the more contemporary sounds of world music to record a number of albums in the 1990s. You can of course hear it today at Scottish and Gaelic festivals, and apparently it's big in Nova Scotia.