On this link you can find a long discussion about Irish strumming :
http://www.thesession.org/discussion...10782/comments
There are two strumming patterns for backing irish jigs (6/8, 9/8, or 12/8) :
for each bar :
| down up down down up down | aka
| dud dud |
or
| down up down up down up | aka
| dud udu |
none is better than the other, you can mix the two patterns along a tune.
If you watch this video of Bothy Band, the founders of irish band music (with Chieftains and Planxty), you can see Donal Lunny (bouzouki) using the first pattern and Micheal O'Domhnaill (guitar) using the other :
http://www.youtube.com/watch?v=kjS2ExtzvII
The same theory fits for backing or for the melody.