I just noticed, that quite often at the end of a song or a phrase in Irish music, the note sort of trails off and gets lower. How is it done? Is there some method, or guidelines for when to/not to do this?
Thanks,
ants
How: just let the air pressure fall and/or increase the size of the mouth cavity.
When: not too much! 
It’s really easy to do in a combination of lightly decreasing the air pressure on the note, and simutaneously begin to slowly finger the next lower note. For example if a song ended on E, you’d play the E then slowly start to cover the D while blowing lighter.
Air pressure alone, will do the job. If you do it in the upper octave, the sound will eventually break into the lower. Joanie Madden does that a lot on some of her slow airs on Song of the Tin Whistle. Roisin Dubh comes to mind.
Tony