Freddy, a slip-jig (nine quavers/quarter-notes to the bar - which may leave you little the wiser!) has a "pulse" of three to the bar, or - to put it more generally - three pulses to the phrase. In other words, an ordinary double jig, as the name suggests, has a feeling of two: i.e. ONE-two ONE-two (think The Irish Washerwoman and imagine yourself tapping along to it in a pub).

In a slip-jig it's ONE TWO THREE, with more or less equal weight (SORry the DAY i was MARRied) - again, if you listen to one and try tapping along to it, you'll probably find yourself falling into that pattern of emphasis naturally.