[2] He succeeded his father in the title in 1564 or 1565, but was a minor, and was placed in the wardship of a relative of his mother, Christopher Barnewall, a politician and landowning knight, of Turvey House, County Dublin.
Plunkett married the 11th youngest daughter (one of nineteen children) of his guardian (and his wife Marion (née Sherle)), Mary Barnewall.
[1]: 29–32 The 7th baron attended a grammar school at Ratoath, and was reputed for his learning; a book by Richard Stanihurst about Ireland, published in 1584, was dedicated to him.
He assisted the government in dealing with a rebellion in 1600, alongside Henry Óg O'Neill and Sir Geoffrey Fenton.
He was acquainted with Queen Elizabeth I, who granted him 20 horsemen and funding to support them, and a mug reputedly from whom has been held at Dunsany Castle since the 16th century.