He moved to Ireland, at the suggestion of his Castlehaven relatives, who had large estates in that country, and himself became a substantial Irish landowner, principally in County Tyrone.
Mervyn played an active part in the proceedings, carrying the Irish Parliament's Remonstrance against Strafford, which denounced his government of Ireland as a tyranny without precedent, to the English House of Commons.
[3] However Ormonde, the Lord Lieutenant, had always distrusted him and preferred to take advice only from the Attorney General for Ireland, Sir William Domville, so that in a few years Mervyn's role as Crown legal adviser effectively lapsed.
He was chosen Speaker of the House of Commons in May 1661 when again member for Tyrone, against the wishes of the King, who would have preferred William Domville.
He was Speaker until the dissolution of Parliament in 1666, although he had greatly offended the King, and his loyalty to the Crown was deeply suspect.
According to the Oxford Dictionary of National Biography, "Opinions concerning Mervyn, both in his own day and since, have been various, but rarely complimentary, with frequent accusations of corruption, lack of scruple, or the pursuit of self-interest above principle".