Tegid, considered the father of the modern Irish race, was actually born in Wales about 300 a.d.
Thus, the peoples of the British Isles are actually an amalgamation of races descended from both natives and invaders.
There were two branches of ancient Celtic: Q-Celtic, of which the Gaelic of Ireland and Scottish Gaelic survive (Manx was a third but recently died out), and P-Celtic, which was everything else (Gaulish, the language of ancient Britain, Galatian, etc.) of which Welsh and Breton survive today. Cornish was in this group but died out 200+ years ago.
Stonehenge was built before the Celtic speakers arrived in the British Isles--there were people living there for thousands of years before the Celts arrived.
As I understand it, Welsh actually derives from the old German root word for foreigner. Ironic given that the Germanic invaders were the foreigners. The self identifying word for the Romano-Britons was ‘Cymri’ and echoes of that can still be found in their western strongholds of Cymru (Wales), Cumberland (the Lakes region) and at more of a stretch, Kernow (Cornwall).
Cornish is now an official minority language in the UK and is growing strongly after dying out in the 60’s. A group of far sighted individuals spent a long time interviewing and taping the last fluent speaker before her death and have used that work to rebuild the language.