[1] He was born in Kinvara, County Galway, the second son of Richard Óge Martyn (died 1648) of Dunguaire Castle and his wife Magdalen French.
His eldest son Oliver Óge Martyn, despite his father's dubious loyalty to the Crown, was able to hold on to the family estates; unlike his brother Peter, he was never attainted, apparently due to the high regard felt for him by his neighbours.
In the general atmosphere of religious toleration which followed the Restoration of Charles II (since both the King and his brother and heir James were known to incline to the Roman Catholic faith) Peter's cousin Patrick Darcy was able to resume his legal practice and may have encouraged Peter to follow him to the Irish Bar.
Ball points out that Martyn had only 14 years in practice; on the other hand, his wealth and social standing may have made him an acceptable candidate for judicial office.
[1] The fact that his brother Oliver was pardoned for his services to James may have given him hope of similarly favourable treatment, but this proved to be an illusion: he was attainted and his estate at Kilconnel was forfeited.