So, Tak, you finally found the O Briain! I tried to buy it from you when you listed it for sale here once, and then you told me you lost it-that was a while ago now!
I’m glad to hear you found it, but it shouldn’t play like that. Do you remember that it was the same way before you lost it? I have one of Cillian’s Low Ds (and played another) and also have a Low Eb, and the low octave is fine on both. They don’t quite honk on the low D and E like a flute, but they have a good volume. So, either it wasn’t made right to start with, or there is something in the windway or tube that is interfering with the air flow. There could be a metal shaving inside, or something else.
Moisture clogging the windway will make any low whistle weaker in the low notes, so you might want to be sure it is fully warm before play, and dipping the fipple in soapy water, and then letting it dry will help to keep condensation from clogging the windway.
If everything is clear inside (make sure there are no insects that crawled in there to hibernate during the time you couldn’t find it), or give it a good soak in something like dishwasher detergent (Cascade Complete with enzyme action) to clean it internally, and if it still is too quiet, then I would try contacting Cillian and see if you could send it back to him for to check it out and see what the problem might be.
One last thought, does the blade at the bottom of the window look damaged or dull? There should be a sharp bevel on it, as there are on mine.
I count my O Briain’s as some of the best low whistles I have had (which were many), with a really nice tone, and very responsive in their playing.
The only time I had trouble is when the moisture affected them-so coat the windway, warm it up well, and then see what happens.
Good luck. Let us know how it turns out.