As a player of both styles, I have found that some bluegrass puctuation is a welcome addition to an Irish session as long as there is no language change. Every style of music has its own language, its own scale limitations, its own bells and whistles. Bluegrass, which is, basically, a wedding of Irish form with Jazz syncopation and melodic variables. The line from one to the other is, I believe, crossed when the improvisation becomes the new melody with nothing in common with the original melody but the chord arrangement. I don't know how things are in Sweden but if your hubby's idea of bluegrass is Earl Scruggs doing "Jimmy Brown, the Newsboy", he will not have to alter his approach very much. If he is more a Nickle Creek kind of guy, he is in for some serious culture shock.
BTW, I have been successful using some Irish techniques with a bluegrass trio. Triplets are a welcome relief to a barrage of sixteenth notes. They have a beveling and humanizing affect. Also, there is a lot to be said for stating the melody, clearly and proudly.