The Lamont harp is usually given a date in the 15th century. I think that the reason for this is the Robertson of Lude family legend, which state that the harp was brought to Perthshire from South Argyll in 1460.
However there is no earlier source for this legend than the beginning of the 19th century, and no independent corroboration of it.
In the absence of the legend, how would we go about dating the Lamont harp?
Carbon dating could be used to get a scientific date range for the felling of the trees used for the wooden parts, but as yet that has not happened. If or when it does, there is not the precision required to securely date the harp - we may not get an answer closer than a century either way. but this would still be a very good start!
We can compare the Lamont harp with the other Gaelic harps. However, these are mostly not securely dated either. But if we group the harps into similar forms, then the Lamont harp clearly does not fit into the "small low headed" or "medieval" group. The medieval harps are the Trinity and the Queen Mary, and these instruments can be compared with the medieval stone carvings at Keills and Jerpoint. Compared to these four examples, the Lamont is rather different. It is larger, with more strings; it is plainer, without lavish medieval carving and painting; it does not have the rounded double headed fish on its forepillar, but instead a stylised flat T-section.
That flat T-section matches that on later harps, for example the Otway harp, which is usually dated either c.1600 or c.1700, and also the Cloyne harp, dated 1621. These two instruments are classified as "large low headed" or 16th-17th century group. The third member of this group is the Ballinderry fittings, the metalwork only from a large low-headed harp. The style of the metalwork and its decoration is similar to the Lamont harp's, with thick heavy cheekbands, fat lobed tuning pins, a metal end cap and a strap to join the neck and pillar. The Ballinderry fittings are not well dated but are usually said to be late 16th century.
So by comparison with the other Gaelic harps, we might suggest that the Lamont harp is perhaps from c.1600. It is even possible that the inscription was added when the harp was new, in 1650.
We should also check the historical images of Gaelic harps. The nearest instruments to the Lamont harp are the instrument in the portrait of William Archdeacon, c. 1750, though this is thought to have been an antique instrument at that time. The other close matches in style are the instruments drawn by Praetorius in 1619 and Timm in 1622.
So it seems to me, from looking at the Lamont harp, that it most likely dates from the first half of the 17th century, and should be classified as a "large low-headed" harp.
What then could be the explanation for the traditional story? Well, perhaps it was in reality the small, highly decorated medieval clarsach at Lude that was brought from Argyll in 1460, but the story got swapped with the large plain one in the late 18th or early 19th century? Or that there was a third instrument that the story refers to?