In Irish literature and folklore the faeries are a mischievous bunch. They have been entrusted with the care of nature and watch out for the forest. However, they are good at making mischief for mankind. Mankind is the enemy for the faeries and forest dwellers because man serves to disrupt their lives by hunting and searching for things that are the faeries.
Fairies were known to steal away babies in the night. The poem talks about the little girl who was stolen away and had died from grief. The fairies live for an endless amount of time, far longer than a man, so they don't understand that the child had died. They believe she is sleeping so they make her a comfortable forest bed and watch over her.
One of the ways that the faeries protect themselves and the forest as well as making mischief is by planting the thorns shrubs. They keep mankind out but when they fail to do so, a man wakes up with them injuring him in his bed. The faeries put them there so that the man will not go back to the forest glen.
In the movie "Willy Wonka" the first lines of the poem are recited by Gene Wilder.