Here's a poem by William Butler Yeats that's full of Celtic imagery. I memorized it long ago, and find it useful on hikes through the woods. It might be a little too lyrical for your tastes, but easy to memorize.

  I went out to the hazel wood,
  Because a fire was in my head,
  And cut and peeled a hazel wand,
  And hooked a berry to a thread;
  And when white moths were on the wing,
  And moth-like stars were flickering out,
  I dropped the berry in a stream
  And caught a little silver trout.

  When I had laid it on the floor
  I went to blow the fire a-flame,
  But something rustled on the floor,
  And someone called me by my name:
  It had become a glimmering girl
  With apple blossom in her hair
  Who called me by my name and ran
  And faded through the brightening air.

  Though I am old with wandering
  Through hollow lands and hilly lands,
  I will find out where she has gone,
  And kiss her lips and take her hands;
  And walk among long dappled grass,
  And pluck till time and times are done,
  The silver apples of the moon,
  The golden apples of the sun.

but there are many...