Wikipedia says the predominant family around Limavady was the O'Cahans, whose mark is found throughout the area. O'Cahan's Rock, in the Roe Valley, is where, according to local myth, a dog belonging to one of the chiefs jumped the river to get help from nearby clans after a surprise enemy attack. This gave the town its name, Limavady being the anglicised Leim an Mhadaidh, the leap of the dog.
Wikipedia's Roe Valley Country Park entry mentions the same legend, of the dog that leaped the River Roe, fleeing enemies or warning the O'Cahans. The legend is the explanation of a place name, and its shape varies.
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