Over the course of the 20th century, Irish playwrights penned scores of adaptations of Greek tragedy and Irish epic, and this theatrical phenomenon continues to flourish in the 21st century. My dissertation examines the performance history of such adaptations at Dublin's two flagship theatres: the Abbey, founded in 1904 by W.B. Yeats and Lady Gregory, and the Gate, established in 1928 by Miche&#225l Mac Liammóir and Hilton Edwards. I argue that the potent rivalry between these two theatres is most acutely manifest in their production of these plays, and that in fact these adaptations of ancient literature constitute a "disputed territory" upon which each theatre stakes a claim of artistic and aesthetic preeminence. Partially because of its long-standing claim to the title of Ireland's "National Theatre," the Abbey has been the subject of the preponderance of scholarly criticism about the history of Irish theatre, while the Gate has received comparatively scarce academic attention. I contend, however, that the history of the Abbey--and of modern Irish theatre as a whole--cannot be properly understood except in relation to the strikingly different aesthetics practiced at the Gate. Unlike Yeats and Gregory, MacLiammóir and Edwards brought extensive professional expertise to their theatre, in which a panoply of color, gesture, music, lighting, artistic and technical design worked in tandem with the spoken word to create the performance's meaning. Their range of technical and artistic genius allowed them to stage ancient literature with lyric passion and cutting-edge visual effects'"memorable barbarities," in MacLiammóir's words. The Gate's achievements radically reinvented Dublin theatregoers' understanding and expectations of theatrical performance, thereby recharting the entire trajectory of modern Irish drama. This study examines productions of ancient Greek and Irish adaptations at the Abbey and the Gate from the beginning of the 20th century through 2006. Utilizing a dual theoretical framework of performance studies and adaptation theory, my study complements close textual analysis with detailed examination of both theatres' archival materials, including playbills, publicity, critical reviews, and visual images of the performances in photographs and video recordings.