One is that there was a huge ammount of cultural exchange between the Celts and the Picts.

The Picts as a culture were wiped from the face of the Earth by the Scoti who were a Celtic tribe. The Picti/Scoti later called "Scots" invented the Kilt.

While the brat + belt = great kilt, I can not find any pan-celtic connection for the phillybeg, it just is not there.

Am I saying that the Irish Catholics can't wear a kilt? No I am not, but neither will I stand and defend them when they claim that the kilt is pan-Celtic.