The summit of Cadair Idris is called Penygadair (head of the chair). It is 893 metres above sea level and marked by a pile of stones and triangulation pillar. Just below the summit is a stone-walled shelter measuring approximately 5 metres by 5 metres, with a former spring providing drinking water in the rocks just to the south-west.
Source: http://snowdoniaguidedwalks.com
M. Ryder, RCAHMW, 5th October 2018.