Crosby, Sir Piers (1590–1646), courtier and army officer, was the son of Patrick Crosby (Mac an Chrosain) and his wife Catherine, probably an only son with one sister. In 1611 he inherited the property accumulated by his father in Queen's County and Kerry, but disputes over his rights saw him travel to England in 1616. Despite a brief imprisonment for unlicensed travel, he was knighted by James I on 17 July 1616, made cupbearer to the king, secured his land titles, and remained at court. His first wife, Sarah, third daughter of Sir Patrick Barnewall and Mary, Lady Barnewall of Turvey, County Dublin, having died, in May 1619 he married Elizabeth Touchet, widow of the 1st earl of Castlehaven and daughter of Sir Andrew Noel and Mabel, Lady Noel of Dalby, Leicestershire. He thereby obtained an interest in former Castlehaven properties in Tyrone and, later, Armagh. In 1627 he took command of an Irish regiment in the expedition against the isle of Rhé off the coast of France. Returned to England the regiment was involved in disorders, and moved on to Ireland in August 1628. Back at court, where he was attached to the circle associated with Queen Henrietta Maria, Crosby had by then become a gentleman of the privy chamber to Charles I as well as being appointed to the Irish privy council. Over the next few years he entertained several schemes to deploy troops in overseas service.
Crosby was granted a baronetcy on 24 August 1630, and in 1634 was elected an MP for Queen's County. As such he clashed with Lord Deputy Wentworth (qv), was briefly imprisoned and removed from the council. His conflict with Wentworth persisted, with Crosby accused of slandering the lord deputy. In 1639, despite mobilising formidable court allies, he was duly convicted, fined, and briefly imprisoned. He took up a command in France but returned to London in January 1641 as a witness at Wentworth's trial. The Irish and English parliaments looked favourably upon his case for redress and he was restored to the Irish council in June 1641. By November 1641 he was serving as an MP for Gowran, Co. Kilkenny, and was commissioned to raise a regiment for government service following the rising. In 1642, however, he travelled to England, and thence to France. Raised a protestant, he was reported a convert to catholicism by February 1643 and in June 1643 he returned to Ireland with arms for the confederates. He appears to have been appointed confederate marshal by 1646. A supporter of the Ormond peace, he was imprisoned in Kilkenny by the opposing faction led by Archbishop Rinuccini (qv) in September 1646 and died there in November 1646. Crosby had at least one daughter, Elizabeth, from his first marriage; she predeceased him.