We meet Barrigan for the first time at the police academy. To quote from various dialogue and directions in the script (remember, Colin = Matt Damon):
POLICE TRAINEES. (INCLUDING BARRIGAN, who is included in all
Colin's trainee scenes). COLIN is in the class, wearing a
trainee's uniform. He has a notebook, a pen. Writing.
Colin: Fucking firemen are getting pussy for the first time in the history of fire. Or pussy.
COLIN sits on a bench looking at THE GOLD DOME OF BEACON
HILL. The terraces of fine townhouses. Aqueous golden light
behind. Misty golden beauty.
Barrigan: What are you looking at? Forget it. Your father was a janitor, and his son's only a cop.
Colin (not vainglorious, but innocently stretching for the idea) You're in trouble if you're "only" anything.
Barrigan: Don't tell me I'm looking at the
first dickhead-American president
of the United States.
Their dialogue actually creates humourous situations at times, such as this moment:
Colin: I don't mind going it alone. If you
could go it alone you might get
Barrigan: We're cops. This isn't "somewhere".
Colin: I know you're a worker. I might be
able to do something for you. You
got any suits at home or you like
coming to work looking like you're
gonna invade Poland.
Colin obviously, at this point, has no idea that Barrigan is a mole (and ends up inviting him onto his personal task force).
So, to answer your first question, yes, we had met him before.
As to your second question, the honest answer is we just don't know. There could have been even more moles in the department who did that. Perhaps Barrigan did it. Perhaps someone else arranged it. We simply don't know. Your question is logical given his duplicity but is ultimately unanswered.
In response to your comment, no, the man Alec Baldwin pushes up against the wall is not Trooper Barrigan. He is simply a camera technician.
Man pushed up against the wall is just in front of Alec Baldwin here:
Trooper Barrigan is here:
So definitely not the same person.