What is the origin of the phrase "Top of the morning to you"?

Each morning, a colleague of mine greets me with the phrase:

Top of the morning to you!

I've tried to figure out what the meaning of this really is and how to properly respond, however there seems to be dozens of interpretations as to what this phrase actually means.

Does anyone know what the origin and original meaning of this phrase is?

The phrase is Irish in origin but now very rarely used in Ireland (except as a sterotypical "Irishism"). It simply means "the best of the morning to you" - perhaps from the idea of unhomogenised milk, where the cream rises to the top. An appropriate response might be a simple "thank you" although the traditional response would be "And the rest of the day to yourself."

Terrible attempts at Irish accents, dancing a jig and leprechaun costumes are entirely optional while saying this.

This was used in Theodore Cyphon, or, The benevolent Jew: a novel, Volume 3 by George Walker, published in 1796. The protagonist is greeted not long after landing on the shore of Essex:

"Halloo ! you teney" cried one, " the top of the morning to you. Have you seen pass a tall chap, in a light blue coat, with striped trowsers."