Linky: http://www.straightdope.com/classics/a5_060b.html
Cecil pretty much focuses on chasing rainbows rather than the pot of gold. The idea seems to have become conflated with the idea of leprechauns’ pot of gold (In the original version, if you could catch a leprechaun, you could force him to tell you where his pot of gold is hidden- the catch being that the leprechaun can disappear if you take your eyes off him for an instant. Or were the legends linked to begin with? That is, was the leprechaun’s gold hidden AT the end of the rainbow, and only the leprechaun could magically guide you there?