Here's a better version of the little poem. I don't know who wrote it; I touched it up a bit for better rhythm:
When IE and EI both say EE,
Who can tell which it should be?
After C, use E then I;
Otherwise IE will apply.
Some exceptions we may note
Which one needs to learn by rote:
Protein, caffeine, weird, and seize,
And in the U.S., leisure, please.