Shannon was once a boy's name and the most popular men's name in Ireland. In fact it is still a boys name. When the English conquered Ireland the occupiers (Orange men granted the lands of Irish citizens to pay off the king's debts) started naming their girl children Shannon to mock and emasculate the Irish. The name spread to england proper and spread with English subjects, however that does not change the fact that the origin of the name as a girl's monkier was an act of bigotry, racism and imperialism. Every male Shannon is a "fuck you" to long dead, self-aggrandizing assholes who'd look their nose down at others.
By the by, this was a standard practice for the English, compare the French Jean.
