Over the course of the late 14^th century and throughout the 15^th century, the O'Donnells of Tyrconnell established themselves as one of the most powerful aristocratic dynasties in Ireland. By the early 16^th century, the authority of the lords of Tyrconnell extended across most of the northern half of Ireland. Their rise to prominence has received a considerable amount of scholarly attention. Much of this has focused on the expansion of O'Donnell power within Ulster—namely the ability of successive O'Donnell chieftains to raise considerable military forces within the lordship of Tyrconnell, which they then used to impose their overlordship upon large areas of Ulster. O'Donnell power and prestige, however, extended well beyond Ulster, and much of their strength derived from their ability to create and uphold a broad web of dynastic alliances stretching across the island of Ireland—a topic which has received very little attention within the historiography of late Medieval Ireland. Moreover, a strong maritime dimension underpinned many of their alliances. The O'Donnells' capacity to raise fleets from among their MacSweeny urríthe (sub-kings, or vassals) within Tyrconnell and their ability to create alliances with powerful maritime kindreds such as the O'Malleys and Burkes of Mayo in Connacht gave them a major tactical and strategic advantage over their traditional rivals, the O'Neills of Tyrone. Drawing upon a broad range of material from within the wider Gaelic world (including Ireland and Scotland) as well as English and Scottish governmental material, this essay explores the maritime dimension underpinning the O'Donnells' rise to prominence during the later Middle Ages, charts the development of the O'Donnell lordship within its maritime context, and demonstrates the importance of maritime power within Gaelic Irish politics during this period.