"if you are backing a tune in D then using DADGAD you can play the tonic chord without including a third, whether major (F#) or minor (F Natural).

You can get the same effect in G when playing in standard tuning,   by playing an F shape on the third fret, but with the second finger lifted, so the G string is played open (of course that assumes you are playing the F shape with the thumb fretting the bottom string, rather than a barré). A very satisfying chord I find that, for some reason.

I've never worked out an equivalent on D with standard tuning. If you drop the D you can guarantee the tunes will all be in G.

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"DADGAD is the bodhran of guitar playing" Basically I feel the essential role of the guitar in Irish sessions, regardless of tunings, is to be a kind of bodhran with strings. It's there to drive, not to lead. And that's a pretty important job.