I think that you may find the following old letters and reference to Patrick Adair that I found in the publication titled, "The Edinburgh History of the Scots Language", Edited by Charles Jones and published by the Edinburgh University Press, interesting:
Page 587 "Another collection in Scots, this time correspondence, is found among the papers of Archibald Edmonton of Duntreath, a native of Stirlingshire who migrated to County Down c. 1604 and settled in Ballycarry, County Antrim, in 1609 with his brother and brother-in-law (Historical Manuscripts Commission 1909). Included are forty-five letters from twenty-eight different members of the aristocracy using some Scots, letters to family members, associates and members of other landed families (such as the Hamiltons, Montgomerys and MacDonnells), and written between 1609 and 1631. Though sometimes addressing intimate family subjects, these letters are generally formal in style, indicating that their writers were schooled in Scots. Two examples addressed from Ballymena follow.
W. Houstoun to [name missing], 25 April 1626, Ballemeanaghe: 'Worschipfull, my deuty unto your self rememberit and not forgetting my deuty and service to my Lord Clanneboyes. Theis ar to put yow in remembrance and also to intreate yow to do me the favour that ye wilbe in Craigfergus on Monday nixt, being May day, that I may haif your contenance thair at the tendur of Abrey Loo his money, as also for my meitting with Jhone McCulloch, becaus ye ar the man chosin for me as it fallis by our submissioun. Sunday nixt was the day of our meitting, bot I think we must put it off till Tuesday next. This hoiping ye will perform as I schalbe ever reddy to yow in the lyik of anything ellis that lyes in my power, and sua remaines, your faithfull freind, V. Houstoun.' (Historical Manuscripts Commission 1909: 123-4)
Sir Robert Adair of Kinhilt to his brother-in-law, Archibald Edmonston of Duntreath, 2 June 1627, Ballemenach.—'Rycht worthie and loving brother, my love remembrit. We expectid to have sine yow longe sence. Ye shall wit that their hes bin servants of my Lord Cheichesters heir taking possesione in Gilform to the use of my Lord Cheichester. We have grait naid that ye and we war together to consult in this bussines and sindry other things. We ar mightilie trublit with the sogers monays daylie in this land, and your not agreement with Mr Houstoune drawis us still in farther inconveniants, as ye will heir at meiting. I have this day reseved letters from Scotland quich urgis me to go over with all the speid I can, quhich makes me the desyrus that we all sould be together befor my waygoing. So thinge els to meiting. With my love to your bedfellow and all our frends, I take leve and still remains, your loving brother to my uttermost, Ro. Adair.' (Historical Manuscripts Commission 1909: 126-7)
These two letters exhibit the same Scottish features as previously cited (for example, each places an -s on remain in the complementary clause where the verb is not adjacent to its subject), but these are markedly attenuated by comparison to the 1571 letter and 1614 land assignment. "
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"On the other hand, that much of the population continued to speak Scots in parts of Ulster cannot be doubted, not only from the fact that Ulster Scots speech areas in the midtwentieth century were almost exactly coextensive with the settlement patterns of Scots in the Ulster Plantation in the early 1600s, but also because of the geographical distribution, as we will see, of dozens of vernacular poets in the eighteenth and nineteenth centuries. Ulster Scots may have been replaced by English in the writing of clergymen, but did not necessarily disappear from their sermons; Patrick Adair (d. 1694) is said to have been accustomed to preaching in it (Killen 1866: xii-xiii). Most likely, spoken Ulster Scots declined in prestige only slowly until the strictures of schoolmasters began to make it an object of eradication beginning in the late eighteenth century and especially in the nineteenth century."