It is generally accepted that James Clarence Mangan developed a pro-Nationalist verse that would later influence literary greats such as William Butler Yeats and James Joyce. Insofar as Nationalist politics were bound up with Catholicism (which, in 19th century Ireland, was usually the case), it would be fairly accurate to say that Mangan accordingly developed a Nationalist Catholic voice. Whether or not his poetry is a form of Irish Gothic, in the vein of such celebrated works as Charles Maturin's Melmoth the Wanderer, is up to interpretation. Based on the disturbing, nearly apocalyptic tone of many of his poems, I would argue that Mangan's poetry at least pays homage to the Gothic tradition, if it isn't quite Gothic itself.
For much of Ireland's history, Nationalist politics (the philosophies that advocated for a free Ireland separate from Great Britain) were often bound closely with Catholicism, as it was the traditional religion of Ireland and had been suppressed, often brutally, by the Anglican British conquerors. Mangan, himself raised as a Catholic, became very interested in Nationalist politics during his adult life. More specifically, he came to believe that Ireland could only win her freedom through violent, armed conflict. His poetry often reflects this idea, and that is why it can be thought of as pro-Nationalist.
Take, for instance, his most famous poem, "Dark Rosaleen." This poem is "translated" from the native Irish language, although Mangan's "translations" were often so loose that he was more or less creating an entirely new work of art. In this poem, the speaker (often thought of as Hugh O'Donnell, the great Irish chieftain) addresses his love, the "Dark Rosaleen," and it is very clear that his love for his subject is intense, nearly obsessive. However, while much of the poem is a relatively straightforward love ballad, the last few lines take a dark turn, describing an apocalyptic event rife with violence and death. It's common to view this departure as a description of the bloody war for Irish independence, and so Rosaleen becomes less of a real woman and more of an allegorical representation of the Irish nation. The dark, cryptic, and disturbing tone of this poem, especially its last lines, is one of the primary arguments for Mangan as a Gothic writer. In any case, the strong Nationalist foundation of the piece as a whole is one of the most obvious examples of Mangan's intense political leanings.