The basic difference between the ubiquitous PC and the Brian Boru jackets are the lapel (BB has a shawl lapel, and the PC a peaked lapel) and the cuff.

The BB has a peaked cuff with the peak in the center of the side of the sleeve, as opposed to the PC which lines up the peak with the seam along the back of the sleeve. Button placement is also different: the BB has either three or five buttons on the sleeve; one at the point, and two equally spaced on either side. Often the two buttons on the rear of the cuff are omitted for reasons (I suppose) of cost-- although I don't see where the savings of four buttons can make much of a difference!

I think you can probably get a decent view of the jacket on the Gaelic Themes website.