I was in Santa Fe, NM, a week ago for my brother's wedding, so while there I tried, without any luck, to find a session to sit in with. The closest thing I found was Celtic de Santa Fe, a dance school; the owner gave me the name of a local musician she knew, but by then it was Thanksgiving and I had to give up.
But I did manage to practice a little. I was playing really badly, and at first I presumed it was because I was somewhat out of practice with all the travelling that I've been doing over the past few weeks. Later on it hit me that I was close to 7000 feet (over 2000 meters) above sea level — higher than Denver by a good bit — and the air was much thinner. It didn't bother me so much when walking around the city or hiking, but I really noticed the difference while playing the whistle.
And that taught me something valuable: My breath control sucks. Or, at least, I wasn't giving it the attention it deserved.
I've thought a lot about breathing in the past, but mostly in terms of intake, how to get the air in. I've noticed that if I inhale too deeply or if I'm running out of air that the quality of the sound I get from the whistle suffers and I need to let some air out through my nose or inhale again, respectively. But when I strarted trying to pay extra attention to keeping my breath pressure consistent it really helped even out the timbre.
But what does "consistent" really mean? Obviously different pressure is required for notes in different octaves, and breath requirement changes across each octave as well. It also changes between different whistles. Rather than think too much about this I'm just trying to listen to the sounds I'm producing and aim for an even tone. This mostly works well, but I notice that I tend to over-blow when I play outside, since the whistle sounds quieter in the open air than it does in the small room where I usually practice. I have to consciously think about playing "quieter" to get it right.