Leading into a cave which is associated with earth in Celtic myth frequently signifies the final stage in a journey. It is the abode of the Ancient Ones whom the Rhymer is about to encounter. He is told not to eat any of the fruits he sees or he will remain in this realm forever which will be to his detriment, showing us that this is a phase to experience for the mortal and it would not be natural for him to stay. He would be back on the Fool's path, only much further along it and with no return possible. However, he is told that his hunger for sustenance will be satisfied with an apple in a short while.
Apples appear to be a common theme within myth also, symbolic of spiritual nourishment.
The hero or character is often led to, or introduced to such apples by an otherworld woman. A main tie in here with Celtic myth is when Niamh brought a branch from the Apple tree of Emhain Abhlach to Bran. The old hag in the story reaches a certain spot and climbs down off the horse and offers Thomas an apple from a small yet perfectly formed tree laden with fruit. She tells him that after eating it he will be graced with the gift of Truth. His endurance once more rewarded.