Laurel, you sure know your Celts, but speaking from the vantage point of taking the Sinclair Tent to all kinds of "Celtic Fairs" and similarly-identified happenings, my strong feeling is that where you see one, it's put on by folks that "have the Celtic" but not the identity of Irish or Scots "clan". They want to have a good time, and usually lack a large enough "committed" support base for a "Celtic" show. Most of them invited Scots clans to set up a tent, not because they necessarily feel that Scots clans reflect any special Celtic feeling, but because Clan tents offer a focus and sometimes a drawing card for people who love the whole idea. In my day, we went gladly to the Celtic venues in Reno, Lake Tahoe, and in some other venues, most of which only lasted a year.
We had a good time and made some new friends, but let's face it, they are not Scottish Games and I doubt that we enrolled many new members. Our Tent Master has cut back on some of the smaller venues but I think he still likes to be around the Welsh, Irish and Scottish people of Celtic persuasion. Some of the ones which have survived are just plain a lot of good Celtic fun.
Johnnye, I have never found anyone who was even a little concerned that Sinclairs got their start in Scandinavia. We have collectively done a great job of mixing blood lines with all kinds of ineresting folks over the past hundreds of years. All of these European- based cultures have generally similar social patterns, that is to say, none of them shrunk heads or consorted with evil spirits, just had a good family time when they weren't out bashing their neighbors to take brides or impress someone with their spirited battle garb (or color, in the case of Picts). You must always check with the sponsor before setting up, in case they have space problems, or are trying to emphasize something that a Clan Tent does not fulfill. Who knows, you may even just go to have a good time!