"Róisín Dubh" is Irish Gaelic for "Dark Rosaleen" or "Little Dark Rose," the name of a patriotic poem disguised as a love song translated from the original Gaelic by James Clarence Mangan.
At the time when the original poem was written, expressions of nationalistic fervor were illegal in Ireland. The Emerald Isle was under the firm control of the English, who regarded Irish nationalism as a serious threat to their colonial rule. So the writer of the original poem got round the restrictions on nationalist expression by cleverly disguising what was a strongly political tract as a love song about a woman called Dark Rosaleen.
Dark Rosaleen, or Róisín Dubh, is a personification of Ireland. In the third stanza, the speaker says that he's roamed over hills and dales for her sake. This is the kind of language we'd expect from a lover in a love poem, demonstrating his undying love by traveling large distances to be at his beloved's side. But of course, the speaker isn't referring to a woman; he's referring to Ireland. His extensive wanderings are not concerned with romance, but with fighting for the cause of Irish freedom.
Later on in the poem, the speaker voices his desire that his Dark Rosaleen shall have the "golden throne," shall "reign, and reign alone". This is arguably the most explicitly political line in the whole poem and expresses the hope of Irish nationalists that their country would one day be free from British control.