The shape of the hilt certainly looks early-mid 18th c., but that pommel looks like the end of a tulip-head pipe drone. How long is the hilt/grip/pommel overall? I've not seen an original with a total hilt assembly length over 4.5 inches. A too-long hilt unit might indicate a latter dirk, or a modern forgery. The pommel, in addition to its shape, seems to have the end of the pommel as a separate plate held on by the tang screw - haven't seen one of those before.

The blade looks like it came from a broadsword (agreed, Dale, that there were some cut-down double-edge broadsword blades used for Highland dirks) and has either a real patina or has been skillfully aged.

Not saying that this dirk isn't a truly old one, but an arms expert should examine the dirk to ensure that you're not paying good money for a forgery. Is there any information on the dirk's provenance? That, too, might help determine if the dirk is the genuine article.