Jacob B. Here is the relavant info from the Ceolas site I provided a link to. It is not as cut and dried as single jigs are written it 6/8 time.

Single Jig: Some confusion exists over the meaning of this term. It sometimes denotes a tune in 12/8. Cranich (1988) classifies single jigs in this way. The reason that this designation is not very common may be that, on the one hand, some tunes in 12/8 can also be thought of as 6/8, thus The Frieze Britches, for example, can be found in 12/8 or 6/8. On the other hand, a tune in 12/8 may sound much like a hornpipe with many triplets. O'Neill's The Quarrelsome Piper, listed as a hornpipe, could be easily written in 12/8. Finally, another category of 12/8 tunes, the slide (see below) exists.

Joyce (1873), Lerwick (1985), and others identify the single jig as a 6/8 tune based on a quarter+eighth+quarter+eighth note pattern and the double jig as a 6/8 tune based on eighth+eighth+ eighth+eighth+eighth+ eighth note pattern. Henebry (1928) generally concurs, although he uses the term jig as others use the term single jig. Breathnach (1971) describes the single jig in similar terms and adds in passing that they may be in 12/8.

Jon