The hero of the song doesn't actually say he was a Whiteboy, only that they were present. On the other hand, he doesn't declare his innocence.
When you the word "buachaill bán" or "buachaillín bán" in other songs, you can't assume a reference to a rural guerilla. The term might just mean a fair (handsome) lad or refer to Bonnie Prince Charlie. I wonder if the Whiteboys chose their name to indicate support for the Jacobites?? It's not implausible. There was certainly a sectarian element in the fighting; most of these societies had Catholic membership, but the Protestant Orange Order derived from "The Peep o' Day" boys. Jim Smyth wrote that the Whiteboys got their name for the white linen shirts they wore. And I think I read somehere that they disguised their faces with flour or covered them with sheets or something like that? Similar secret societies went under other names such as the Ribbonmen and the Hearts of Oak. and the Hearts of Steel.
The most significant gains for tenants rights (including acts which enabled them to become small proprietors) were won in the second half of the 19th century (i.e., after the famine years). I think it fair to say that the rights were won by the civil disobedience campaigh of the Land League, though historians might argue about the relative importance of rent nonpayment and boycotts (Capt. Boycott was a landlord), of parliamentary activity (Charles Stuart Parnell), and of sporadic acts of violence. Land League leader Michael Davitt had previously been involved in the Fenians, a physical force movement, and decided a new approach was needed.
There are two articles from an Irish nationalist perspective, about the famine, with some reference to the Land League at http://inac.org/history/hunger.html