In Moore's placenames (see my site) he states:
It will be noticed that most of the simple names have the definite article, either in English or Manx, prefixed. - somewhere he or Kneen (or maybe Gill has a more learned discussion on this but couldnt immediately find it - something also to do with norse/gaelic names

Guilc, Guilcagh (K), ‘ Broom.’ In THE GUILCAGH, ‘ The Broom.’ This is the name of a farm, which was probably so called from the quantity of the broom plant growing there. [(Irish) GUILCAGH.]