Adam Cusack (c.1630–1681) was an Irish barrister and judge of the seventeenth century.
He was born in Rathgar (then a village, now a suburb of Dublin city), the second son of Robert Cusack of Rathgar Castle (which Adam inherited on the death of his elder brother) and his wife Alice, daughter of Sir George Sexton of Limerick. He was the grandson of John Cusack, Lord Mayor of Dublin 1608-9. He was educated at Trinity College, Dublin and became a fellow of the college in 1654. He entered Lincoln's Inn in 1655, was called to the Bar in 1660, and entered the King's Inn in 1661. His choice of the law as a profession may have been influenced by the fact that James Barry, 1st Baron Barry of Santry, a leading barrister who became Lord Chief Justice of Ireland in 1660, was his father's cousin.
He was appointed second justice of the provincial court of Connacht in 1661 and became the last Chief Justice of Connacht in 1670. On the abolition of that office in 1672 he was appointed a justice of the Court of Common Pleas (Ireland). In the relaxed political atmosphere of the early 1670s his Anglo-Irish background and tolerant attitude to Roman Catholics were not a political disadvantage. He had also the advantage of having married Catherine Keating, sister of John Keating, later to be Chief Justice of the Irish Common Pleas and niece of Maurice Eustace, Lord Chancellor of Ireland.
The only possible objection to his appointment to the Bench was on the grounds of ill health: from early middle age onwards he suffered badly from gout, which became so acute that he was unable to perform his judicial duties for at least two years. He died, aged only about 50, in 1681, and was buried in St. Audoen's Church, Dublin. In his will, which, according to Elrington Ball, shows his kindly and charitable nature, he left money to the poor of St. Audoen's parish and of Rathfarnham, for the relief of poor prisoners, and bequests to the Bluecoat School at Oxmantown and to the army hospital at Back Lane, Dublin. The bulk of his estate was left to his widow Catherine, who remarried his cousin Nicholas Cusack; she and Adam had no children.
Rathgar Castle fell into decay in the eighteenth century, and was a ruin by 1769, when the Dutch-born artist Gabriel Beranger did a watercolour which shows two men surveying the remains. The Castle probably stood on what is now Highfield Road, at the present site of St. Luke's Hospital, Rathgar.