Assuming your renfest allows attendees to wear/display weapons such as dirks (some don't allow anybody to carry weapons except staff members who are acting as fest characters) and are portraying an historical Highlander, then I'd recommend a mild steel dull/blunt dirk to avoid somebody grabbing it and then suing you for $5 million after he/she cuts him/herself.

If you are wearing a kilt as part of modern Scottish dress, you don't need a dirk, and instead of a sgian dhu I'd recommend a sgian brew (you'll need it).

The only time I carry a dirk is when I'm attired in 18th c. Highland dress as part of a reenactment event. My recreated dirks are sharp (the way the makers finished them) and occasionally come in handy as tools (cutting ropes, for example), but I keep them close and don't allow anybody not in my reenactment group to handle them due to liability issues.