ChiangMai is Lao Lanna and was be part of Anachack Chiangthong ( Luang Prabang ) and was ruling by King SayaSethaThiraj before he came Back to Chiangthong then to build Vientiane as the capital of LaneXang.
Only Thai people don't want them to be Lao, but they call themself as Tai Muong is Luang Prabang language. Tai Moung is for all Lao people who live in ChiangMai or Lanna, but their original natives are Lao.
I have many relative in ChiangMai and Chiangrai, they speak more Lao than Thai, they have teir won letters by using " Arksorn Thamma " intead of Thai alphabets. They also love peolpe to call them Lao Lanna or Khon Muong according to most of my realives told me. Any time I travel to Laos I have to go to visit them in ChaingMai and Chiangrai.