Sometimes you'll see jigs notated in 12/8 rather that 6/8 time. That doesn't make much difference to the accompanist. Just think of each 12/8 bar as two 6/8 bars like this:

equals

Tempo and Feel

Today we often talk about single jigs and double jigs. In practice this is mainly a concern for the soloist and the dancers. The rhythm playing is more or less the same for both types although the double jig is usually a bit slower and heavier than the single.
The Irish Dancing Adjudicators Examination has specified the jig tempo to 72-76 beats per minute for a double jig and 112-120 for a single jig. This is definitely a bit too slow if you play without dancers. 130 is perhaps better.
The jig feel is pretty straight, perhaps with a slight delay of the offbeats.

Rhythm patterns

The easiest way to acompany a jig on the banjo is to simply play the beat - two (or four in 12/8 jigs) downstrokes in each bar.

That rarely sound very good though. We really ought to add the offbeats on the third and sixth eight note too, like this:

Play the beats with a downstroke and the offbeats with an upstroke. This gives the jig a lot more "bounce."

Single notes

The banjo is a loud instrument, so strumming across all strings all the time may sound a bit too heavy though, especially if there's just one or a few instruments playing the melody. The solution is to lighten up the downbeats a bit. Play just one of the strings (the fourth or third) on the downstroke, and two or three (first, second and possibly thrid on the upstroke like this:

Now we're getting somewhere! We're not going to go into details about alternating bass this time, but if you want to, you can expertiment yourself with changing between the third and fourth string for the downbeats.

You can also try playing a full chord on the downbeat and a single note on the offbeat. It's not nearly as useful as the other way round, but it may come handy.

Offbeat only

Sometimes even a single note on the downbeats can be a bit too much. The downbeats are usually quite well epmhasized in the tune and by other chord instruments, so maybe we should just leave it out alltogether:

This variant is mostly useful if there are other chord instruments playing as well, but don't rule it copletely out even if your banjo is holding down the chords alone.

Summing it up

You've probably noticed that although there are six or sometimes twelve eight notes to the bar in a jig, we nearly always think of them in threes. In a 6/8 jig way play two groups of three eight notes in each bar, in 12/8 we play four.

The patterns we can use, are:

Downbeat only
Down and up
Offbeat only
Single note on the downbeat
Single note on the offbeat

Nos 3 and 4 are perhaps the most useful for us since the first two can get a bit too heavy on a banjo. (although they sound great on a guitar and other softer sounding chord instruments) while no. 5 sounds almost identical to no. 1

In addition to these common patterns, there are some others we can use for variety. More about that later.