To answer some of these:
* The Diatonic can play a full natural scale over one octave - ie all 8
notes. Outside this, a tune may fit, or may need bending (or even harp
swapping) to play.
* "Danny Boy" fits on a diatonic harp played "straight", all except for one
note at the end of the verse, which needs a bend.
* "Paddy Richter" is an alternative tuning, which has the 3-blow taken up a
semitone, so on a C harp, instead of having 2 Gs, you get a G and an A,
thus
avoiding the bend required on a standard harp
* Brendan Power (who posts to these lists) is one supplier of this tuning,
but it is one of the more common custom tunings.
http://www.brendan-power.com/
* This isn't the only way to play this tune, but is one of the most
discussed.
* If by "the National Anthem", you mean "God Save The Queen", it can be
played straight on a standard harp, by starting on 4-blow, ie the root note
for that harp. If you want to play it in the key written, ie G, that means
you will need a G harp.
Mike