Will you be playing with or without a drummer? If you are playing with a drummer it is almost certain that you can afford to play in a very economical style which gives the music a nice open feel. You will be able to play a mixture of mainly on the beat or sometimes off beat but resist the temptation to be jazzy by playing slightly in front or behind the beat.

If you are playing without a drummer, the dancers will rely on you to get the pulse for the dance, for all the counting to eight and sixteen that dancers need for their patterns, so keep it simple, resist any urge to go and play wifty wafty melodic stuff, just emphasise the beat and to add some occasional interest find the natural run up or down to the next chord change. The I V stuff can seem cheesy, but use it as part of an arpeggio going I V octave-I and then it seems less so. As has been mentioned, lots of the time will be spent playing in the keys of D and G so get cracking on those scales to find some alternatives to just playing I V. Persude some of the other musicians (especially on guitar, accordeon/melodeon) to put in some relative minor chords or some flattened sevenths to spice up the accompaniment. There are some tunes (eg Winster Gallop) where it is fun for the whole band to start playing in a minor key - instant Klezma!

I would suggest not playing open notes. The reason for this is that as you play a note, you can lift your finger slightly from the finger board thereby muting or damping the note. This gives a nice precise sound to replace the missing drumbeat and it is this kind of precise sound that dancers need for ceilidh dancing.

Fingers or plectrum? Although I use a plectrum for playing guitar and bouzouki but I've never felt comfortable using one for bass where I use my index and middle fingers, mainly alternating, but I suggest you use what feels comfortable. Tone will be important too. Avoid using a very deep or muddy sound, try to achieve a balanced sound, one that still has some warmth but still with a bit of edge, otherwise the definition for the beat will disappear and that means curtains for the dancers and also as others have said, don't stray into the octaves where you are competing with the other instruments.

Finally, get out and see some other ceilidh bands and try to analyse what the rhythm section is doing, what makes it good or bad and then apply that to your own playing. If you go to the Webfeet you can then check out some samples of other ceilidh bands, again to try and analyse what the bass player is doing.

Good luck.