I'll try to help you out... I'm just learning how to transpose songs. It's a bit of a challenge to figure out at first, but it starts getting easier as you do it. I wish I had paid more attention in Music Theory.

For the songs written in the key of G: just play them with the normal chord fingerings, but with the capo at the third fret. That will change your overall key to B flat. (Bb)

If you have printed sheet music (for piano) you can tell what key it is in fairly easily:
Here are some of the most common ones-
1 sharp - key of G
2 sharps - key of D
3 sharps - key of A
4 sharps - key of E
1 flat - key of F

This site may help you as well : http://www.cyberfret.com/misc-wisdom/capo/page6.php

Hope this is helpful. Good Luck!
PS - Excellent song selection!

Thanks for the help Troy L its very much appreciated to take the time out to reply.
I found the website link very useful but the whole transposing 'thing' is still a bit confusing.
If this is of any help i took a look at the music i have for the songs and noticed -
The boys of Bluehill
2# (sharps?)
1 on the top line and 1 in the second space down
The Blackthorn stick
1 #
1 on the top line
The Greencastle Hornpipe
1 #
1 on the top line
The Belfast Hornpipe
4 #
1 above the top line,1 on the top line,1 on the second line down and one in the second space down
The Harvest home
2 #
1 on the top line and 1 in the second space down
Saint Patricks Day
1 # on the top line
Cronin's Hornpipe
1 # on the top line

(These # symbols are just after the treble clef at the start of each line)

Glad you like the song collection.Thanks again.

Bob,

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.

PJ