The town of Enniskillen stands on an island in the River Erne, and its Irish name is Inis Ceithleann, island of Ceithleann or Cethlenn. Wikipedia says Cethlenn was a figure in Irish mythology who may have been a goddess. Local legend says she was wounded in battle by an arrow and tried to swim across the river that surrounds the island, but she never reached the other side, and the island was named in memory of her.
The Lough Erne Pilgrim Way includes a story called How Enniskillen Got Its Name among its Fermanagh folklore tales. The only description available to me is its one line intro, so the version above follows Wikipedia.
The name has been written in many ways over the centuries, including Iniskellen and Inishkillin, which is the root of the name of the regiment of Inniskilling.
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