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 you are probably aware, Cornish, Breton and Manx still survive in the academic realm of ancient language departments.