When learning to backup celtic music i went through alot of different guitar picks trying to find what worked for me. For the past few years i have been using a Dunlop Nylon .73mm Its stiff enough to give a good sound, but not so much that i push the strings sharp when i am strumming. Different players use different types. When i saw Tony Mcmanus playing backup he was using a super stiff pick. John Doyle uses a Dunlop .60mm. I think when compared to bluegrass players celtic guitarists tend to use a thinner pick. Instead of a boom chuck hitting seperate strings i tend to strike all the strings at once, which is easier with a thinner pick. For me a heavier pick tends to push the 6th string sharp, especially in Orkney capoed at the 2nd frett.
There are no hard and fast rules, and it all comes down to personal preference. Something worth thinking about though.