In Noah's voice-over during the drug factory flashback, he says "While the wicked stand confounded, call me, with thy saints surrounded." In the first The Boondock Saints (1999) movie, that line is painted above the Irish flag on the wall in the arms dealer's warehouse.
When the brothers meet Rocco at the bar in their dream, a black cat walks past Rocco across the counter. The cat is a sly reference to Rocco's girlfriend's cat Skippy, who Rocco accidentally shot in the first The Boondock Saints (1999) film.
The Boondock Saints II (2009) was released ten years after the original film, The Boondock Saints (1999), but the events of the sequel are said to take place eight years after the events of the first film, putting them in the year 2007.