Just to add another level of intrestto the whole Kilt/Celt discussion, I just got this from Answers.com Early History of Ireland page.
http://www.answers.com/topic/early-history-of-ireland

"The truth is more complex. For a start, recent DNA studies have suggested that the people who introduced the Celtic languages to these islands may well have been Celtic-speakers, but they were not members of a Celtic race. Ethnically they were indistinguishable from the pre-Indo-European inhabitants who preceded them. What’s more, their arrival had so little impact on the genetic inheritance of the native peoples that they cannot have numbered much more than a few thousand.

The Y-chromosomes of the modern Irish are closely related to those of the Basques, which has led some anthropologists to surmise that the Basques are a remnant of the pre-Indo-European population of western Europe, and that the pre-Celtic language (or languages) of Ireland may have been related to Euskara, the Basque tongue. (See Celt for a discussion of the so-called “Celtic problem.”)"

So if the Irish aren't really Celtic, genetically speaking, then either are the Scots, right? Keeping in mind this is regarding genetics, which is not necessarily the same thing as cultural. Could be the Irish and Scots are genetically of a pre-Indo-European race but adopted aspects of Celtic culture.

It all gets so complicated.