When Connor and Murphy acquire their ammunition, the wall reads, "While the wicked stand confounded, call me with thy saints surrounded". This is the English translation of the Latin words "Confutatis maledictis, voca me cum benedictis" from a Latin religious hymn by Franciscan monk Tommaso da Celano.
The tattoo that Connor has on his left hand/index finger says, "VERITAS," which is Latin for "Truth." Murphy also has a tattoo in the same place but on his right hand, that says, "AEQUITAS," which is Latin for "Justice/Equality."
The church where Connor and Murphy attend Mass, Church of the Covenant, is not a Catholic church, but a Protestant one. No Catholic churches in the area would allow filming in their sanctuaries due to their objection to the plot of the film.
The woman that the priest refers to in the opening scene of the film, Catherine Genovese, was an actual person. She was raped and murdered by a serial rapist in 1964 outside her home in Queens while 38 eyewitnesses did nothing to save her. This apathetic bystander phenomenon is now referred to as the bystander effect or "Genovese Syndrome", stemming from the inability to take action or to know whether action has already occurred. In both The Boondock Saints and the graphic novel Watchmen (2009), the story of Kitty Genovese inspires a vigilante to go to war against crime.
When first entering The Sin Bin the words "Abandon all hope, ye who enter," are painted on the Door. This is a reference to literary masterpiece "The Divine Comedy" by Dante Alighieri. When the character Dante enters hell in The Divine Comedy, the same words are carved at the entrance.
Was given a very limited release in the United States due to the Columbine Massacre. It was only shown in five theaters for only one week. However, it would later develop a large cult success from word of mouth following the success of DVD sales and would follow up with the sequel The Boondock Saints II: All Saints Day (2009).
The script's initial sale garnered a considerable amount of publicity (including the cover of USA Today) as a "rags-to-riches" story; writer/director/composer Troy Duffy was a bartender at J. Sloan's in Los Angeles when Miramax head Harvey Weinstein not only bought the script, but signed Duffy to direct, his band to score the film and agreed to purchase the bar for Weinstein and Duffy to co-own. However, Duffy quickly managed to sour the deal, putting the script into turnaround where it was eventually produced for less than half of the budget offered by Miramax. After its limited theatrical release, the film gained popularity on home video as a Blockbuster Exclusive, unfortunately Duffy's contract did not give him any royalties from video sales. Duffy's initial success and consequent self-destruction are chronicled in the documentary Overnight (2003).
Connor and Murphy both use suppressed Beretta 92f pistols throughout the movie, with Rocco's small revolver being a .38 caliber snub-nose. The .38 Snub-Nose is a Colt Python. The revolver is really a .357 magnum, but both the mag and .38 can be shot out of the handgun.
The bar patron in the overalls and green bandanna in the scene where the McManus brothers get into the bar fight with the Russians. Duffy is actually seen standing in the background and doing nothing after the fight starts. In fact, his whole band (The Boondock Saints, formerly The Brood) have cameos in that scene, including Troy's brother, Taylor Duffy.
The trivia items below may give away important plot points.
In the Sin Bin, Connor says, "I've been waiting for this one," referring to one of the two additional victims. The comment refers to an omitted scene in the emergency room, where the man is a pimp that was beating a prostitute, and Murphy holds Connor back from attacking the pimp. The other victim in the Sin Bin is a drug dealer from outside the Sin Bin (yet another omitted scene).
The line spoken by Il Duce as a narrator "When I whet my flashing sword and my hand takes hold in judgment, I shall take vengeance upon mine enemies and I will repay those who hate me" is in the Bible at Deuteronomy 32:41.