In the early decades of the seventeenth century, intermarriage of the Ulster Irish with the newly arriving Scots, and conversion of the Irish of Ulster to Presbyterianism, seem to have been common. In her book, The Catholics of Ulster (Basic Books 2001), Professor Marianne Elliot implies that much of the Catholic gentry disappeared from Ulster, not because they were exiled and dispossessed by their Protestant neighbors, but because they were converted.
A study by G. B. Adams concludes that a significant number of the native Irish population of east Ulster converted to Presbyterianism shortly after the early infiltration by the Scots—even before the plantation of Scots. Adams, in his article Aspects of monoglotism, at page 84, says:
"a considerable part of the old irish population [in east Ulster] seems to have been absorbed into one or other of the reformed churches, usually into presbyterianism, which in the seventeenth and early eighteenth century must have had a considerable Irish-speaking membership. The association of Irish language survival with strongly Roman Catholic area belongs to a later period and to central and west Ulster."
Aodh de Blacan, in The other hidden Ireland, in Irish Studies, xxiii (1934), pages 439–454, supports the same conclusion:
"The schism of the sixteenth century cut geographically across the Gaelic world, Scotland and that part of Ulster which was infiltrated, not planted, became Protestant even before the plantation of Ulster."
The last two citations and quotations are from The Irish Language in County Down, by Ciaran Devine (Ciaran O Duibhinn), which is chapter 17 of Down—History & Society edited by Lindsay Proudfoot (Geography Publications 1997) (page 438).
Charter schools are an example of efforts made at converting Catholics to Presbyterianism in Ulster. The schools were entirely Protestant in management, and the children were reared as Protestants. History of the Catholic Church: From the Renaissance to the French Revolution by Rev. James MacCaffrey, S.J., 1914 (volume II, chapter X, The Penal Laws). In 1734, The Incorporated Society in Dublin for Promoting English Protestant Schools in Ireland was established by a charter of George II.
Other sources indicate that a substantial number of native Irish Catholics in Ulster converted to Presbyterianism. For example, see Ower the Sheugh:
"There is evidence however that a number of 17th century Scots settlers took the Gaelic language with them to Ulster. In his book 'Presbyterians and the Irish Language', Roger Blaney quotes evidence supplied by various historians and states that many of the lowland Scots settlers in Ulster were probably bi-lingual in both Scots and Gaelic.
"The Reverend James Stothers in his unpublished work, 'The Use of the Irish Language by Irish Presbyterians' comments that a significant number of the Presbyterians who settled in Ulster, spoke Gaelic. These Presbyterian immigrants to Ulster would have continued to use their Gaelic language at least for the first generation and possibly longer.
"There was a policy of recruitment and encouragement by the Synod of Ulster of Irish speaking Presbyterian ministers of both native Irish and Scots origin, e.g. Jeremiah O’Quinn and James Wallace. Stothers comments that with the presence of such Ministers and a large number of Gaelic speaking Scots settlers present, the conversion of many Irish Catholics to Presbyterianism is easier to understand.
"There would not have been the same cultural disparity felt between the native Irish and the Scots settlers as there would have been between the native Irish and the English settlers.