Finnin MacCarthy (Irish: Fínghin mac Donncha Mac Carthaig) (1560–1640), was an Irish clan chief and member of the Gaelic nobility of Ireland (Irish: flaith) of the late 16th-century and the last credible claimant to the Mac Carthaig Mór title before its suppression by English authority. Mac Carthaig's involvement in the Nine Years' War (1595–1603) led to his arrest by the Crown, and he spent the last 40 years of his life in custody in London. His clan's lands were divided among his relatives and Anglo-Irish colonialists.

Mac Carthaig was born in 1560 at Kilbrittain Castle near Kinsale in the province of Munster in Ireland, into the MacCarthy Reagh dynasty, rulers of Carbery, the son of Donogh MacCarthy Reagh, 15th Prince of Carbery. His grandfather was Donal MacCarthy Reagh, 12th Prince of Carbery.

The significance of Mac Carthaig's career lies in his command of territories in west Munster, at a time when the Tudor conquest of Ireland was underway. Southwest Munster was the area most open to Spanish intervention, which had been mooted from the late 1570s to aid Catholic rebellions in Ireland. The overlord of much of this area, but excluding Carbery, was the MacCarthy Mór of Desmond, whose lands were located in modern west Cork and Kerry. There were, in addition, three more princely branches of the MacCarthy dynasty, the MacCarthys of Muskerry, the MacCarthys of Duhallow, and finally the most wealthy: the MacCarthy Reagh of independent Carbery, of whom Florence's father had been a (semi-)sovereign prince. It was into a complex interplay between the crown government and these opposing branches that Florence found himself pitched.

The Mac Carthaig Reagh branch established itself as loyal to the crown during the Desmond Rebellions (1569–73 and 1579–83), to assert their independence from their nominal overlords, the Earl of Desmond and the Mac Carthaig Mór, both of whom had joined the rebellion. Mac Carthaig's father, Donnchadh Mac Carthaig Reagh, served the crown faithfully and reported that he had mobilised his men to drive the rebel Gerald FitzGerald, 15th Earl of Desmond out of his territory during the Second Desmond Rebellion. When his father died in 1581, Mac Carthaig, by then in his late teens or early twenties, led around 300 men in the English service with the assistance of an English captain, William Stanley, and his lieutenant, Jacques de Franceschi, under the overall command of the Earl of Ormonde. They drove Desmond's remaining followers out of MacCarthy territory, 'into his own waste country', where the rebel earls' troops could find no provisions and deserted. Mac Carthaig also claimed credit for the killings of Gorey MacSweeney and Morrice Roe, two of Desmond's gallowglass captains.

Upon his father's death in 1581, Mac Carthaig inherited substantial property but was not the prince's tanist (second in command and usually successor to the head), and therefore did not assume his father's title, which went to Mac Carthaig's uncle, Owen MacCarthy Reagh, 16th Prince of Carbery. The position of tanist went to Mac Carthaig's cousin, Donal na Pipi (Donal of the Pipes). But, in 1583, Mac Carthaig did go to court, where he was received by the queen, who granted him 1000 marks and an annuity of 100 marks. In 1585 he served as a member of the Irish Parliament at Dublin.

Upon his marriage to Ellen, the daughter and sole heir of the Mac Carthaig Mór (also Earl of Clancare), Fínghin mac Donncha fell foul of the crown government in Munster on account of the prospective unification of the two main branches of the Clan Carthy. To add to government suspicion, there was also a rumour of communications by him with Spain. In particular, he was accused of contact with William Stanley and Jacques de Franceschi, who had defected with a regiment of Irish soldiers from the English to the Spanish side in the Eighty Years' War in Flanders.

As a result of these suspicions, Mac Carthaig was arrested in 1588 as a precaution against his assumption of the title of Mac Carthaig Mór, which would have given him command over huge estates and thousands of followers. The English authorities considered this too dangerous a prospect in a country they were trying to pacify and disarm.

Initially detained by Carew at Shandon Castle in Cork, after six months Mac Carthaig was moved to Dublin, and then to London, where he arrived in February 1589 to be committed to the Tower. His wife escaped from Cork a few days later, probably on his instructions. Mac Carthaig was examined by the privy council in March and denied all complicity in the continental intrigues of the English Catholic, Sir William Stanley. He was sent back to the Tower, but fifteen months later his wife appeared at court and Sir Thomas Butler, 10th Earl of Ormonde, volunteered to stand surety for him in the sum of £1000. Since no charges were proved against him, Mac Carthaig was set at liberty in January 1591 on condition that he not leave England nor travel more than three miles outside London without permission.