I would suggest that you go with what you are comfortable with, in the sense that if you normally wear jeans, wear what you would wear with them. My feeling is that a sportkilt is too light to wear to such an event although they are common at Highland games where the competitors wear them.
My experience is that Celtic events are laid back and even if you do see other kilts there, they will be closer to t-shirt and sandals. Ham is right though, most sandals are off-balance with the kilt so short socks and Doc Martens would be more appropriate.
I also think that the Jacobite shirt doesn't usually fit, I'm not sure, My wife wants me to wear one.
Anyway all people see is the kilt so there you go. If you're looking for the formal answer, you would be dressed too formal for almost anything.

Relax, crash and burn and learn for next time. Make it part of you and not a third leg appendage thing.