So topical these days. Well shucks I play both and tell you the truth I never gave it that much thought.

A good rule of thumb that saves a lot of time and flapping around, find the principal beats and be on the harmonic best BASS note on the Guitar, the strums most of the time are on an Up or Offbeat so they would not matter that much except:

You are playing the modern Celtic wallop chording style which I absolutely despise, too old I guess. Buy a book ...

Other helps, find the predominant note, in Folk Dance D E G F C and rarely Bb, form mediant-less harmonies, nail the strings to it or retune, then all you need is a strong right hand to keep the rhythm going.

Other helps for fanatics, me included, a Gobiron and Harness where you can merrily play both the tune and strum the Guitar at the same time.