Today (17th March) is St Patrick's Day, called in the Irish language Lá le Pádraig. St Patrick was a missionary in Ireland in the 5th Century. Although his historicity is not in doubt, a great many legends about him occur. At least one has a cryptozoological element.
There is, in County Donegal, a lake called Lough Derg (Irish, Loch
Dearg, the red lake). Legend had it there was a monster in the lake. In one form of the monster legend there was a prehistoric lady called the Hag of the Finger. A monster emerged from her thigh bone. It looked like a hairy worm. The hero Conan threw it into the lake and it became a monster. St Patrick compelled the monster to go to the bottom of the lake, where yet it is said to linger. Another legend says Patrick put a curse on this lake, so no salmon are found in it.
A famous tale is that St Patrick drove the snakes out of Ireland, but, as far as we can tell, there were no snakes in Ireland before his arrival. Snakes just didn't care to swim across the sea. For that reason, there are no snakes in Iceland or Greenland either. A Greek writer named Solinus mentioned Ireland's snakelessness before Patrick was born.
Though people nowadays wear shamrock on St Patrick's Day, the custom may not be very ancient. It is first recorded in 1681. Tradition had it that Patrick used it to explain the Trinity. The custom of holding parades on St Patrick's Day originated in North America.