O'Toole, Fiach (c.1525–1578), Gaelic lord, eldest son of Art Óg O'Toole (qv), was an influential figure among the Leinster Irish and the marchers of Dublin. The first mention of Fiach is from November 1546. Then Art Óg's lands of Fartry, which he had held of Henry VIII, were confirmed to him. Fiach and his cousin Brian O'Toole (qv) (Brian an Chogaidh) of Powerscourt and Fercullen were to endeavour to live as loyal subjects, the latter having held the office of sheriff of Co. Dublin during 1545. Fiach and his brother Murchadh were pardoned on 10 July 1549 for their involvement in the murder of a Richard Rely by Robert Roche of Dengyn, and this misdemeanour did not do Fiach any lasting damage. In February 1551 he received his livery as the heir of Art Óg.
He, like other Leinster nobles, was increasingly concerned about the punishment being meted out by the government to the O'Mores and O'Connor Falys of Laois and Offaly. Indeed Fiach's concern was evidenced by the fact that he had taken some of the refugee O'Mores into his service. Moreover, he moved closer to the rapidly rising Aodh O'Byrne (qv) of Crioch Raghnuill (d. 1579), as evidenced by the latter's marriage by 1550 to Fiach's sister Sadhbh. His discontent with the government was to manifest itself during May 1556 in an open act of defiance. With his cousin Feilim O'Toole (qv) (d. 1603) of Powerscourt and Fercullen and the O'Tooles of Imaal, Fiach joined the brothers Diarmait Kavanagh (d. p.1570) and Brian MacMurrough Kavanagh (qv) (d. 1578) in an attack upon the Pale. Their advance into the Pale was checked and they were driven back to Powerscourt by Sir George Stanley, the marshal of the army. At Dublin seventy-four of the raiders were executed publicly, but high birth saved Fiach and the leaders from the gallows; they obtained their pardon on 8 June.
In July 1557 the freeholders of the Glencap uplands of north Wicklow brought a case before Lord Deputy Thomas Radcliffe (qv), earl of Sussex, against Fiach and his cousin Feilim of Powerscourt, complaining that open conflict had emerged between them for the right to levying dues upon the inhabitants of Glencap. As Glencap fell within the jurisdiction of the sheriff of Co. Dublin, Sussex ordered both Fiach and Feilim to desist from levying their traditional dues, but it is unlikely that this decision had any real effect on them. Fiach continued to develop his influence among the marcher families of Dublin, fostering close connections with the Archbolds, Talbots, Walshes and Harolds. Like his cousin Feilim, about 1557 he took a wife of settler stock, marrying Rice Basnett, the daughter of Dean Edward Basnett (d. 1545) of St Patrick's cathedral. The inclusion of a James Greene in Fiach's pardon of 11 August 1559 is also evidence of good relations with the Dublin marchers. These links between the O'Tooles of Castlekevin and the marchers were further evidenced on 3 October 1560, when William Basnett, John Walsh and Reamain Harold witnessed Fiach's grant of Glassinvokyn to an Edmund Archbold. Fiach's influence among the marchers was no doubt assisted by the fact that he was later sheriff of Co. Dublin. It was on this support base that Fiach was to lean heavily during his occasional strained periods with the government.
In July 1564 Fiach, with the Harolds and the O'Tooles of Imaal, caused trouble along the borders of the Pale, earning themselves a pardon on 3 August 1564. He and his retainers were again troubling the borders of the Pale during the summer of 1566; he obtained another pardon on 16 August and was bound to appear before Sir Francis Agard (d. 1577), seneschal of the O'Byrne and O'Toole countries. Thereafter Fiach seems slowly to fade as a major Irish noble in east Leinster, as the star of Aodh O'Byrne and his son Fiach O'Byrne (qv) (d. 1597) rose. But Fiach of Castlekevin was sure-footed enough to move with the times, forming a new alliance with his O'Byrne kinsmen. This alliance was sealed by the marriage of his daughter Rose O'Toole (qv) to Fiach O'Byrne about 1571–2.
The alliance pleased Fiach of Castlekevin and Aodh O'Byrne, but the former clearly had a difficult relationship with his new son-in-law. On 11 February 1575 Fiach of Castlekevin gave a statement to government officials investigating the dealings of Gerald Fitzgerald (qv) (d. 1585), 11th earl of Kildare, with the Leinster Irish. The tale Fiach of Castlekevin told was one of his son-in-law's collusion with the earl in the orchestration of a programme of extortion throughout western Wicklow and Kildare. Specifically he accused the earl of authorising Fiach to attack the Wesleys of Bishopscourt, Co. Kildare, and of encouraging him to destroy much of Castledermot, Co. Kildare, in late 1572. It is likely that Rose O'Toole and Aodh O'Byrne saved Fiach of Castlekevin from retribution, dissuading his now very irate son-in-law from that course. It seems that this row is referred to in one of the poems contained within the ‘Leabhar Branach’ (the poembook of the O'Byrnes): there the poet appeals to the two Fiachs to renew the old friendship of the O'Byrnes and O'Tooles. Even so, relations between the two men remained fairly sour. The government also seemingly enticed Fiach of Castlekevin further into their fold by appointing him sheriff of Co. Dublin, most probably in 1576. His old habits remained, though: he and his son Seán were pardoned for unspecified offences on 21 April 1577. Fiach died 4 April 1578, leaving his lands to his eighteen-year-old son Barnaby O'Toole (qv).