This is an epic series of Celtic fantasy novels by Katherine Kerr. I am close to finishing (soon-ish) all fifteen books and I can’t say enough good things about this series and it’s talented author. I can’t even begin to describe the complex woven story lines, so I’ll just leave you all with some links that explain it better than I ever could. Possible spoilers ahead – but it won’t at all take away from reading this sprawling and detailed series.
https://en.wikipedia.org/wiki/Deverry_Cycle
http://www.deverry.com / http://www.deverry.com/student.html#deverry