The term Dirk is also apparently used as a generic term for daggers from the 171th and 18th century which I suppose is probably where most the confusion comes from.
The Highland Dirk is the only one I know of that has a short blade. The British Royal Navy, Soviet Union, and Japanese Imperial Navy all had Dirks with longer blades.
From what I got from *cough* the dictionary *cough*, a dirk seems to be a 'short dagger of a kind formerly carried by Scottish highlanders'.
But either way it's a dagger, and can really be used by both bandits and naval officers, and probably has been.
The Highland Dirk is the only one I know of that has a short blade. The British Royal Navy, Soviet Union, and Japanese Imperial Navy all had Dirks with longer blades.