A lot of tree/plant names have had their application changed over the centuries, particularly in Welsh (they get applied to local plants unrelated to the original but sharing some particular attribute). I read a paper a while ago arguing that the original Celtic word for rowan was a significant name element but was now applied to quite different and unassuming plants in Welsh and Irish.
Holly is celyn, From Proto-Celtic *kolinnos (compare Old Irish cuilenn m (“(wood of the) holly-tree”), Cornish kelyn, Breton kelenn). The old British/Irish *Colinos is found in several names in different forms (the i causes varying degrees of umlaut and breaking of the o), from a late 5th century inscription "CUNORIX MACUS MAQVI COLINE" (macus and maqui represent different cases of the same Q-Celtic [Gaelic or Celtiberian) word for "son") and to the names of early "
Saxon" kings Ceaulin and Ceol (in contemporary terms kings of the Gewisse). In Welsh though it has switched genders.