Rapparees were dispossessed members of the Gaelic warrior aristocracy, driven out onto the hillsides to make way for incoming colonies of Protestant English and Scots settlers. They survived by raiding those same settlers and ambushing soldiers or government officials whenever possible, so they were something in between bandits and true guerrillas- but then again, most guerrillas are.
Young Ned is supposed to have fled to the hills after shooting a tax collector, possibly in defense of a virtuous old widow- although the more mythic the details, the less reliable from a purely historical perspective. There he lived for several years, coming down only to strike and then escape again, before he was eventually killed. The rapparees were generally Jacobites- supporters of the exiled House of Stuart- but they became a symbol of Irish resistance for the later (and politically quite distinct) Irish Republicans. The real Young Ned of the Hill would probably have found the socialism and egalitarianism of Irish Republican politics to be peculiar if not offensive to him as a born aristocrat, but as a symbol of a heroic Gael standing up to English oppression, he can still be pressed into service regardless!