In 1633, Thomas Wentworth became Charles I’s representative (Lord Deputy) in Ireland. Some of the Presbyterian ministers such as Robert Blair, who had been forced out by the bishops, asked Wentworth for help, but it soon turned out that he hated Presbyterianism. The Presbyterian ministers went back to Scotland, but many people from Ireland rowed over in boats to hear them preach and take communion, usually rowing back home the same day. When the National Covenant was signed in Scotland in 1638, thousands of people signed it in Ireland as well. In 1639, Wentworth, tried to force all the Presbyterians in Ulster over the age of 16 to sign the ‘Black Oath’ against the covenant. If they didn’t sign it they would be fined or put in prison. Some signed it, but many fled to Scotland rather than obey.

Read more:
Thomas Hamilton, History of Presbyterianism in Ireland (Belfast, 1992 [1887]).
James Seaton Reid, History of the Presbyterian Church in Ireland, ed. W. D. Killen (3 vols, 3rd edn, Belfast, 1867), i, 210ff