After the low D, what do you consider to be the most useful low whistles for playing Irish trad. music?
Maybe a low A (if you consider that a low whistle). Plays D major well.
That depends on the repertoir and musicians you play with (at sessions for fun, or recordings for fun). I agree with Dale’s low A suggestion. The low A works great with fiddle repertoir; which to me, are the great tunes (in D major, B minor, A major, and A modal - help fiddle enthisiasts) that dip below D and have a two octave range.
I have recently come to appreciate the utility of the low G, as the new session I play at has a harper and a flute player that play the harp repertoir quite a bit (Carolan & Bunting tunes). Before this session, I really questioned the versitility of the low G whistle in sessions.
Bartleby,
I noticed you’re in N.E. Alabama, I suspect that the Irish Trad. in your area is going to have a seasoning of Appalachian - Old Time, and maybe a little Blue Grass filtering down from Tenn. A lot of Old Time fiddle pieces are in the key of A, so you might want to consider the Mezzo-soprano/alto A. This is the one between regular and low D’s.
If your playing with singer’s in your area, you might also find that a number of tunes need pitched a little lower or higher to accomodate alto singers singing a tune designed for a soprano. Here a Low F may help out. It plays in F and Bb and in a range that a lot of Alto and Second Soprano voices prefer. For example when your guitarist capo’s up 3 frets and plays a tune in g, he’s playing it in Bb.
If you start to accompany a singer on your Low-d (key of d) and they ask if you can raise it a bit, You can move it to F by playing the D fingering on the f. If its still abit low you can go back to the Low-D and play the tune in the key of G. If it’s still to low you can take the fingering for the key of G and move to the f whistle where it will be the key of Bb.
Of course you can just give in to your WhOA and get all the key, 
Regardless, it all how you …
Lee I think that would be a first fret capo, the open E (6th string)would become an F and the A (5th string)would be Bb. I know I’m missing something here though so maybe you can help me out. I agree the F whistle is a great choice. Highland Bagpipes are mostly tuned to Bb so the F is wonderful for getting that sound. Old Blind Dogs are from Scotland, and I think Rory plays his F more then anyother key. F is also great for jazz and blues.
Outside of trad music, the E is great for playing with guitar. Many blues players tune thier guitar to an open E. Slide players love the open E tuning, as well as Gmaj which is often called “Spanish” the Hawiians call it “Slack Key”. Spanish tunning is popular as well in blues. Ry Cooder uses this tunning most of the time.
If it’s just one key that you can get, I would go with the F before the A. Just for the diversity of keys. If you can get two keys, A & F will get you into many places in music. They may also get you kicked out of many places as well though. 
[ This Message was edited by: jackorion on 2002-04-11 22:23 ]