According to the Collins Welsh Dictionary ‘derwyddol’ means ‘druidic(al)’ from ‘derwydd’ meaning ‘druid’. Deri is a group of oak trees, derwen is a single oak tree, dôl is meadow
As happens often in languages the -r- gets moved for ease of pronunciation, cf. Old English ‘brid’ became modern English ‘bird’, Catalan ‘cabra’ meaning goat became ‘craba’ in Aranese/Gascon.