Declán mac Eircc (also Déclán or Declan), Declanus in Latin sources, was an early Irish saint of the Déisi Muman, who was remembered for having converted the Déisi in the late 5th century and for having founded the monastery of Ardmore (Ard Mór) in what is now Co. Waterford. The principal source for his life and cult is a Latin Life of the 12th century. Like Ailbe of Emly, Ciarán of Saigir and Abbán of Moyarney, Declán is presented as a Munster saint who preceded Saint Patrick in bringing Christianity to Ireland. He was regarded as a patron saint of the Déisi of East Munster.

The main source for Declán's life and cult is a Latin Life or vita, which however, survives only in a redaction of the late 12th century. It is witnessed by two manuscript texts which Charles Plummer has shown to derive from the same original: (1) Dublin, TCD, MS E.3.11 (dubbed T by Plummer), f. 66b-71d; and (2) a somewhat more damaged version in Dublin, Primate Marsh's Library, MS V.3.4 (Plummer's M), f. 101 ff. These two manuscripts are also collectively known as the Dublin Collection (or the Codex Kilkenniensis, though the name is also used to refer to the Primate Marsh's Library manuscript only). In its received form, the Life leans heavily on the Life of St Ailbe of Emly in the Codex Salmanticensis, but earlier materials may have been incorporated. The introductory chapters draw on early Irish sagas, notably the origin legend of Déisi related in the Expulsion of the Déisi and the story of Lugaid Riab nDerg in Aided Meidbe and Medb's Three Husbands. Declán is emphatically designated as a bishop of the Déisi, which appears to echo the monastery's political ambitions in the 12th century, when the Irish Church was reformed into a diocesan system (see the Synod of Rathbreasail and Synod of Kells). Ardmore aspired to the status of episcopal see in the new diocese, but the privilege went instead to Lismore, founded by St Mochuda.

Murphy is an Anglicized version of two Irish surnames: Ó Murchadha/Ó Murchadh ("descendant of Murchadh"), and Mac Murchaidh/Mac Murchadh ("son of Murchadh") derived from the Irish personal name Murchadh, which meant "sea-warrior" or "sea-battler". (Muir meaning "sea" and cath meaning "battle").

Spelling variations include Murphey, MacMurphy, MacMurrough, O'Morchoe and others. In modern Irish, "Ó Murchú", rather than "Ó Murchadha", is used.

Murphy is the most common surname in Ireland and the fifty-eighth most common surname in the United States.