More info from the same site:

The first true dirks appeared in the early 1600's, evolving from the medieval ballock dagger. The ballock dagger was a stabbing weapon, designed to pierce armor, with a heavy, sharply-pointed blade and a handle in the form of an erect penis with the testicles forming the ballocks, protective bulges between the handle and blade. In polite company, these were sometimes called kidney daggers.

The first Scottish Dirks retained the ballocks between the handle and blade but developed a wide, flaring pommel capped by a circular brass disk. The handle was carved of ivy root or boxwood root, usually cylindrical in shape, with grooves or one or two bands of Celtic knotwork carved around it. The blade was 12-13 inches long, thick, heavy and triangular in both profile and cross-section. Visitors to the Highlands during this period commented on how thick the dirk blades were and how sharp the single edge was kept. Quickly, the entire surface of the handle began to be richly carved with deep Celtic interlace patterns.

At first glance the large, flaring, disk-topped pommel looks awkward; but it served a purpose. When carrying the targe and dirk, the flared pommel helped prevent the dirk from being knocked from the clansman's hand in the shock of combat, when the targe was receiving blows. Under these circumstances, the shape of the pommel made sense.