Being at a meeting in Cork in 1667, he, with others, was arrested by officers who came to break the meeting up, and was sent to prison: though the Magistrate, who recognized him as the son of the lord of Shangarry Castle, offered to set him at liberty if he would give his word "to keep the peace," which he refused. From the prison he addressed a letter to the Earl of Ossory, giving an account of the arrest and imprisonment of himself and friends, showing their innocence, and pleading the liberty of conscience demanded by the precepts of the gospel. An order was immediately dispatched by the Earl for his release; and as it was soon noised abroad that Admiral Penn's son had turned Quaker, the Earl wrote to his father, communicating the information. Startled and annoyed by the intelligence, the Admiral ordered William to come home immediately, which he did. Josiah Cole, a minister in the Society of Friends, met him at Bristol; accompanied him to London, and being deeply interested for his stability and preservation, went with him to his father's house. Fully as William had adopted the principles of Friends, and many as were the baptisms he had already passed through, he had not yet adopted the plain dress that distinguished them from others; and his father observing this, and that his rapier still hung by his side, hoped that his friend the Earl had been wrongly informed; and he treated him and his friend during the evening with ordinary courtesy, without alluding to the report that had reached him.