Just deleted my previous post because I changed my mind (

). I was leaning more toward a late 19th c. dress dirk with celluloid grip, but then I saw this little gem-(sorry, Moderators, I'm at work and can't post the pics direct from this computer)
http://landandseacollection.com/id582.html
This is a very reputable site and I trust their judgement, but perhaps yours is a little later, maybe mid-19th c. Note the blade dimensions, shape, and short triangular scabbard with single ring like yours. Even the hilt is similar and probably ivory. These types of shorter naval dirks favored by American naval midshipmen, but possibly as another branch of military side dagger. Very nice find-