Oh jeez, who could remember all this stuff?

Here's what we do: if a tune's in the key of G or D, we sometimes drop our E string to a D. We practice using it on chords with the note D in them (such as G, D, Dm, Bm) and practice not using it with chords with E in them (such as A, Am, Em, Em, C etc.) Except for that one string, your familiar chords and finger positions are unchanged. We don't press the frets on the low D string, because the whole idea was to get a nice, rich low D in the song. So why raise it?

DADGAD seems to be for guitar as percussion instrument. Use it sparingly. Also, if you wish to imitate the style of the modern "powerhouse Celtic band",make sure you have health insurance to cover tennis elbow.