The surname Mountain might seem a strange Ulster surname, but is simply an anglicised form, in this case a straight translation of the Gaelic surname Ó Sléibhín, or descendant of Sléibhín. Sléibhín is a diministive of Sliabh (a mountain). In Ulster, the name of a branch of the Cinel Eoghain. Ó Sléibhín is also anglicised as Slaven. In the Ulster Heritage DNA Project there are several Mountains families that match Slaven families. When researching one's family history it is always helpful to know both the Gaelic and various anglicised forms of a surname.