To play along with a Bb whistle, your best bet is indeed to capo the 3rd fret.
Problem is, with the whistle, you basically play tunes in the first and fifth ( doh and fah ), which is Bb and Eb ( or G and D on guitar with capo on 3rd )
In the songs you want to play, you can pretty much take it that the root key is the Start/End chord displayed, i.e. D for 'Bluehill', G for 'Blackthorn'.
These chords would seem to be intended for playing on a D whistle, where the 1st and 5th would be D and G.
Sooooo...... you're either gonna have to capo on the 8th fret, and play in D for the D tunes and G for the G tunes, or transpose the chords you have shown, moving the D tunes up 5 semitones to G, and the G tunes up 5 semitones to C, then play Bluehill in the newly converted G key, and Blackthorn in the newly-converted C key ( still with capo on 3rd ), while your friend plays Bluehill in his root (Bb or Doh) key
and Blackthorn in the 5th ( Eb or Fah ) key.
I realise all of this sounds complicated, but if you read my simple guide to transposition:
http://www.guitarsongs.info/transpos.htm
.. do the simple maths and change all of the chords so that Bluehill will be... G C D G D Em etc.
and Blackthorn will be C G C G C Am etc., when you play these with capo on 3rd, you should be spot on to accompany the tunes on Bb whistle.
