In the final courtroom scene, the guy in the screenshot below sneaks over and triggers a fire alarm in the courthouse. Who is he?
The answer to this question is: Detective "Dolly" Alapopskalius.
He is the character that I found to be credited with pulling the fire-alarm in the script to the sequel (of Boondock Saints): All Saints Day.
This character is portrayed by actor David Ferry in both films.
While the copy of the original script I found has the fire alarm being pulled, it makes no mention of the character that pulls it. Further research of All Saints Day (Boondock Saints 2), reveals that this script has an important clue in the opening sequence of the script (on page 7):
Dolly, dressed as a civilian, pulls the fire alarm.
This sequence recaps the end courtroom scene from the first movie.