Don't know much about history but I am learning more and more each day about my own heathen ways! Here's a bit of info for you on shamrocks:

Shamrock - The Celtic trefoil, which originated in the east. Pre-Islamic Arabs called the trefoil shamrakh, the three-lobed lily or lotus flower of the Moon-goddess's trinity: a design of "three yonis" which appeared on artifacts of the ancient Indus Valley civilization, as well as on stone, pottery, and woodwork in Mesopotamia, Crete and Egypt between 2300 and 1300 B.C.E.

Christians pretended* that St. Patrick explained the doctrine of the Christian trinity to the Irish by exhibiting the shamrock. However, the Irish were worshipping this emblem of their Triple Goddess long before Christianity appeared in their land. It stood for her triple "door," and her God sometimes bore the title of Trefuilngid Tre-rochair, "Triple Bearer of the Triple Key," a trident representing the triple phallus. He was known as a God of the Shamrock, partially assimilated to Christianity by a legend that he appeared to the Irish on the day of Chri's crucificion, bearing sacred stone tablets and a branch with three fruits.

-taken from The Woman's Encyclopedia of Myths and Secrets, written by Barbara G. Walker, published by Harper and Row, San Francisco, 1983.

* I personally feel the use of the word "pretended" is inappropriate and negative in its connotation as it is used here. I think what the author is trying to say is, simply, that the Christians created their own myth and legend about St. Patrick to suit their needs.

Now go on out there and enjoy yourself a tall, frothy green beer...or three of them, should you see fit! I'm staying in and drinking green tea instead!
My husband is apparently having an I.T. meeting at work today, at the Celtic Tavern downtown. SHYAH RIGHT! Meeting, my ass! Hope he doesn't get shloshed!

My thanks to Lili for pointing me in the direction of this historical explaination of St. Patrick. I learn something new every day!