St. Columba (also known as "Columcille") was born of the O'Donnell Clan in Garten, County Donegal, Ireland, on December 7, 521; he died on 9 June, 597. In between, he founded monasteries in and evangelized Ireland and Scotland. The story of his meeting Nessie -- said to have taken place in A.D. 565 -- was first recorded by Adamnan in "The Life of Saint Columba" sometime in the late 7th Century. An excerpt:
How an Aquatic Monster was driven off by Virtue of the Blessed Man's Prayer.
On another occasion also, when the blessed man was living for some days in the province of the Picts, he was obliged to cross the river Nesa; and when he reached the bank of the river, he saw some of the inhabitants burying an unfortunate man, who, according to the account of those who were burying him, was a short time before seized, as he was swimming, and bitten most severely by a monster that lived in the water; his wretched body was, though too late, taken out with a hook, by those who came to his assistance in a boat.
The blessed man, on hearing this, was so far from being dismayed, that he directed one of his companions to swim over and row across the coble that was moored at the farther bank.
And Lugne Mocumin hearing the command of the excellent man, obeyed without the least delay, taking off all his clothes, except his tunic, and leaping into the water.
But the monster, which, so far from being satiated, was only roused for more prey, was lying at the bottom of the stream, and when it felt the water disturbed above by the man swimming, suddenly rushed out, and, giving an awful roar, darted after him, with its mouth wide open, as the man swam in the middle of the stream.
Then the blessed man observing this, raised his holy hand, while all the rest, brethren as well as strangers, were stupefied with terror, and, invoking the name of God, formed the saving sign of the cross in the air, and commanded the ferocious monster, saying, "Thou shalt go no further, nor touch the man; go back with all speed." Then at the voice of the saint, the monster was terrified, and fled more quickly than if it had been pulled back with ropes, though it had just got so near to Lugne, as he swam, that there was not more than the length of a spear-staff between the man and the beast.
Then the brethren seeing that the monster had gone back, and that their comrade Lugne returned to them in the boat safe and sound, were struck with admiration, and gave glory to God in the blessed man. And even the barbarous heathens, who were present, were forced by the greatness of this miracle, which they themselves had seen, to magnify the God of the Christians.