O'Dempsey, Edmund (d. 1658), catholic bishop of Leighlin, was the son of Terence O'Dempsey, Viscount Clanmalier, and probably his first wife, Mary, daughter of Sir Maurice Fitzgerald of Lucagh, Co. Kildare. Clanmalier was a powerful catholic landowner whose 22,000 acres at Clanmalier straddled the borders of King's and Queen's counties. Edmund became a Dominican and studied arts and theology at the University of Alcala de Henares, near Madrid. In late 1634 the Irish chapter of the Dominicans elected him their provincial, a post he held until 1638. Soon after his appointment as Irish provincial, the master general of the Dominicans conferred upon him the degree of bachelor of theology. From 1636 he used his connections to lobby for the bishopric of Leighlin, and was provided to this see in spring 1642.
During the 1640s he played a prominent role in the catholic confederacy, being a member of the confederation's supreme council in January 1648. From 1645 he was a vigorous supporter of GianBattista Rinuccini (qv), papal nuncio to Ireland, and along with the nuncio opposed the proposed Ormond peace of 1646. When the general assembly of the confederacy debated the nuncio's decision to reject the Ormond treaty in January 1647, O'Dempsey acted as the leader of the nuncio's party in the assembly. He declared that the confederates would simply have to reject the peace and God would do the rest. In May 1648 Rinuccini's excommunication of those who adhered to the proposed truce with protestant forces commanded by Murrough O'Brien (qv), Lord Inchiquin, split the hierarchy and the confederation. O'Dempsey never wavered, rigorously enforcing the censures in his diocese. In September 1648 the general assembly of the confederation at Kilkenny, then controlled by Rinuccini's enemies, wrote to O'Dempsey, demanding that he appear before them and threatening to confiscate his diocesan property. He replied that he would not travel to Kilkenny out of fear for his own personal safety.
As the Cromwellians overran Ireland in 1651–2, he tried to prolong the catholic resistance in his capacity as vice-metropolitan of Leinster. In September 1651 he presided over a synod of the Leinster clergy at Ballydrohide, Queen's County, which approved of attempts to persuade the duke of Lorraine to intervene in Ireland on behalf of the catholics and which attempted to re-establish the catholic confederacy. He presided over another Leinster synod on 25 May 1652 which excommunicated those catholic forces who surrendered to parliament. However, the synod's authority to promulgate such censures was contested and they had little effect.
Probably soon after, O'Dempsey fled to Spain, where he settled at the Dominican convent in Madrid. On 27 August 1653 he was empowered by the pope to grant absolution from Rinuccini's 1648 censures to those who showed themselves to be sufficiently penitent. Unhappy with his treatment in Madrid, he had relocated to the Dominican convent of Pontevedra in Galicia by November 1656. He died there soon on 27 August 1658 (O.S.). He wrote a manuscript entitled ‘Feed your flock’, but it was lost en route to Louvain where it was to be published.