Yes, Rocky Road to Dublin is a slip jig, and can be sung quickly enough to dance to - though hopefully not the same person singing and dancing.
Bach's Jesu Joy of Man's Desiring is also in 9/8, and is almost all eighth notes. This is probably a coincidence, but one of these days I'm going to try playing it with a jig-accented rhythm and link it to a fiddle tune... that would be fun.
Sorcha, regarding your statement: "I think I remember being told that when doing a slip jig, the dancer(lady) slips sideways along a straight line, rather than staying in one place (solo performance) or moving in a circle (contest, aka "feis" jigging). Anybody know for sure?'
This is not the case according to my experience. In general, the hard shoe dances are relatively stationary and the dancer mostly faces one direction, whereas the soft shoe dances cover more floor space and involve facing more directions. But I haven't noticed any difference between the slipjig and the soft reels or jigs along these lines. In any rhythm of soft shoe dance, some steps are in place, some go straight forward or backwards or sideways, some are circular, and some follow more varied paths (e.g., straight diagonally then a half circle back home). My influences have been three Irish/Irish-descended women teaching in central Canada, one of whom learned from one of the others; maybe in Ireland or in other circles it's different.
It is customary for a solo dance to begin with a circular "lead-around" that is quite simple. However, I danced in a feis once and the lead-arounds were always omitted because for each rhythm you compete in, you get only 32 bars, 16 of which have to be the mirror image of the other 16, so all the dancers want to show their more complicated stuff.
And I have a question for those discussing single/double jigs. Does anyone know what a "treble jig" is, musically? In Irish dancing we called a jig done in hard shoes a treble jig. I asked my teacher, who is not a musician (or rather, not an instrumentalist) what made a certain tune a treble jig, and she didn't know except to say that treble jigs are slower than "light jigs". Does anyone know if there is an inherent difference between treble jigs and other jigs, besides their speed? Can any jig be played at treble speed or light speed?