Hardly any deep lake in Ireland is without the occupancy of an uncouth monster, seldom seen on the surface, but often observed in motion, far down in the depths of the water. Imagination is frequently exercised by the peasantry, to divine its exact form, which is supposed to differ in species from that of any known denizen of the deep; and mystery shrouding its functions, the Peistha is dreaded as a malignant demon, always bent upon mischief, and especially towards the human race.

The boatmen cast many a furtive glance downwards, to see that it approach not within stroke of their oars, as they greatly desire to give it a wide berth. Any provoked hostility on their part is apt to procure a dangerous retaliation; and in his frequented waters, the Peistha is believed to be master of the situation.

At one time St. Molua, who travelled much through Ireland on his mission of founding churches, visited the ancient Drumisneachta, now Drumsna, in the County of Monaghan. In a neighbouring lake, he saw two boys swimming. But advancing on them, and with a forefront large as a boat of considerable size, appeared the monster of that lake, as if about to devour them.

Not wishing to terrify the boys, he shouted to them:
“Swim, my boys, with all speed towards me, so that I may reward the one who shall first arrive, and that I may know who will swim the fastest.”

Both struck out towards him, and soon gained the shore. No sooner had they landed, than Molua reached them his hand. Having landed safely, they looked back towards the lake, and were greatly frightened at seeing the monster, which had almost gained upon them.

Immediately the saint, raising his staff, struck the Peistha with it on the breast, and it gave a terrific roar. Filled with excitement and alarm, one of the boys died on the instant. However, the holy man prayed for him, and afterwards he came to life.

But a malison he pronounced on the Water Sherrie, ordering it to return and to remain under the waters of that lake. Thenceforward it was condemned never to molest man or beast to the Day of Judgment.