When Catholicism arrived in Ireland, there had been a strong Celtic presence for centuries. The Celtic religion, Druidism, was based in nature. Mountains, storms, rivers and seasons were all significant in Druidism. Then Saint Patrick arrived.
Patrick used a three leafed clover to explain the Holy Trinity, his attempt to teach the Celts that Polytheists were a step lower on the ladder to heaven than Monotheists. The stem united three people in one God. Patrick explained that the leaves of the shamrock/clover represented Father, Son and Holy Spirit.
The Celtic traditions had such a stronghold in Ireland, Catholic priests decided to mingle existing Celtic feast days, beliefs and customs with Catholic ones in an attempt to sway the indigenous group's beliefs more easily and readily.The four-leaf clover is an example of Celtic myth and Christianity intermingling. That fourth leaf represents luck. Superstition had no place in this new religion called Catholicism.
Right: Artist's imagining of the Celtic festival of Beltane on May 1st. The sun grows warmer, and crops and cattle will hopefully flourish.
There are many examples in Irish stories and culture of both the old religion and the new co-existing. My mother was a devout Catholic. On May 1st every year she hung egg shells from a tree. Beltane, which means good fire, is the Celtic festival that welcomes back the warmth of the sun. The warming sun assured Celts that agriculture would be bountiful. The fertility of crops and cattle being important to subsistence farmers, as were the Celts.
The four-leaf clover legend is that if you find one, you will have luck; white clovers are considered to be even more lucky, due to their rarity. Historically it is believed that ancient Celts warded off evil with clovers.
The generation before me put four leafed clovers between the pages of their prayer books! The assimilation of Celtic mythology into Christianity is most apparent in Irish folklore.
In the myth, The Children of Lir, the swans are converted into their original human forms when they hear the ringing of a church bell for the first time.
Although I sometimes think that Patrick, and missionaries like him, destroyed a unique culture; the truth is that we would know very little about Celtic history unless Catholic monks recorded oral myths such as Tain Bu Culaigne. The "Tain" is known as the most complete Irish epic myth and recounts the tale of the cattle raid of Cooley.
This assimiliation by the Catholic missionaries is a double-edged sword really. The Celts were going to be assimilated into the Roman Empire, maybe those on the fringes of Europe might have survived, some in fact did; but the assimilation into the Christian faith meant destruction of the old ways, while recording the history of an oral tradition. An oral tradition was preserved due to religious invaders and their ability to keep a written record of events.
Left: The Celts
If you are interested in the Celts and their lives before Christianity arrived in Ireland, check out my book, "Mona The Body in the Bog."
Bottom image from http://www.fullissue.com/index.php/history-of-the-celts.html