In two articles in recent editions of The Ulster-Scot newspaper, Nelson McCausland has dealt with the issue/myth that a substantial proportion of the first Scottish settlers who came to Ulster in the 1600s spoke Gaelic. There is virtually no evidence to support this claim.
Here's an example of the writing of the period. Rev Josias Welsh was the minister at Templepatrick in County Antrim. He was John Knox's grandson. This is a letter he wrote to Anna Montgomery, Countess of Eglintoun in Ayrshire, on 19 October 1632:
Madame - I have made bold to writte these few lynes to your ladyship, hauyng the conueniencye of this bearer. I confesse my neglect in this duetye; but truelie my indisposition and waunt of ane heart fit for anye good duetye hath beene my hyndraunce: but now, dead as I am, I aduenture, and speciallye beeying encouraged with good tydyngs that I haue to writte to your ladyship, which I know wil be as refreshyng as cold watters to ane wearye and faint person, to wit, the Lords worke prospereth gratiouslye in this countrey; it spreadeth abroad (blessed be His name!), and notwithstandpg the great opposition it hath, it flourisheth indeed lyke the palme tree : and euen the last Sabath in Antrim, ane English congregation, the superstitious forme of kneelyng at the sacrement put away, and the true paterne of the institution directlye followed, which was ane thyng that wee could neuer looke for in that place. It is true the worke hath beene opposed and sore set too, but, blessed be His Holines, it hath done no harme but good; for now the Lord worketh more in one day then in ten before; and where they flocked before, they flocke ten tymes more; sua that in this little church, Sunday was senyght, wee had aboue 14 or 15 hundreth at the sacrement; and neuer such ane day had wee from mornyng to nyght, without faintyng or wearines (prayse to His name). Such motion I never saw; new ones commying in that neuer knew Him before.
Your ladyship shal be pleased to marque Gods wisdome, that since the Bishop beganne to question us, there is, I dare say, aboue three hundreth that God hath taken by the heart that neuer knew Him before, and this within this 7 moneths : upon this condition long may we be in question, and neuer may the Bishop rest. And blessed be the Lord moroeuer, His wisdome hath sua disposed the matter that stil the scourge hath beene shakyng ouer our heads, and neuer remoued, but the execution delayed from tyme to tyme. He wil not let be laid on yet, hauyng ane respect of the weakenes of some who hath need to be better strengthned yet, and that others may yet be brought in ; neither wil be remoued altogether, and sua keepeth the people a flocht, and giueth them not leaue to settle in securitye, but maketh them greedye to use their tyme wich is allotted them : and sua our God is wise, and turneth their courses against themselfs for the furderance of His owne cause. Wee haue gotten tyme yet til May day, and that unexpectedlye contrarye to theer purpose ; and I hope more good wil be done in this tyme, then all the malice of both diuels and men will be able to undoe.
As for you, elect ladye, what shal I say to you but what the Apostle sayeth to the Thessalonians, I. ep. 5 ch. 24 v., Faithful is he that hath called you, that wil also doe it; and thynk not straunge that you be exercised with tryals within and tryals without : most you not be baptised
with the baptisme wherewith your Lord was baptised; if rare for grace, why not rare for crosses also? The Lord keepeth that wyse proportion with His owne : if you haue gotten the gold, will you not get the fyre also? I am of the mynd that yet greater tryals are abydyng us : The Lord prepare us and make us readye. Now I most draw to ane end, hauyng troubled your ladyship with these confused and undigested lynes onlye to show my duetye to your ladyship, wich, if they be seasonnable or acceptable, I shal endeaoure myselfe ofter in this.
Now the God of all grace, who hath called you to the fellowshyp of His Sonne, and endowed you with rare grace, after you have suffred ane whyle, make your ladyship perfect, stablish and strengthen you ; and with my dayly poor wysses to that effect for your ladyship, I rest
Your humble seruant in the Lord,
Mr Josias Welsch
Templepatrick, this 19th of October 1632.
(Taken from the Memorials of the Montgomeries, compiled by Sir William Fraser, 1859. It is also quoted in full in the footnotes on page 18 of The Macdonnells of Antrim by Rev George Hill (1873). In Noble Society in Scotland by Keith Brown, it says that Anna had been converted in 1629 and "...became an active supporter of dissidents in Ayrshire and Ulster, using her household to promote their religious views..."