Try not to get distracted by that sublimely beautiful rendition of Eleanor Plunkett, and listen to Dearbhail Finnegan playing Fanny Power.

Eleanor Plunkett, Fanny Power, Planxty Irwin

You will notice that while playing FP, Dearbhail is tapping her foot twice per bar which gives a clueregarding the above discussion - 3/4 or 6/8 time? She would be tapping three times per bar if she thought it was a waltz time.

I found Dearbhail Finnegan's video by searching for Caitriona Rowsome, after Richard Mellish's recommendation of her book. My search led me to a harp duet performed by these two ladies, and then I found the Fanny Power video.

(By the way, if I search on Google for Fanny Power, the "intelligent" search keeps wanting to suggest Fanny Pack and not Fanny Power.)