He is mentioned in the book, "Memorials of John Gregg," by Robert Samuel Gregg, in the NY Public Library, on p 1, as being the father of Richard and Thomas Gregg, all of them of Cappa, Ennis, Co Clare, Ireland.

The following is from the book, "Corporation Book of Ennis," Brian O Dalaigh, editor, ISBN: 0-7165-2469. Page numbers in () refer to pages of this book:
1721 - On 24 Jun, a presentment was made to reimburse Jonathan Gregg what he expended in providing carriages for the army; and to be allowed for insolvencies and to procure a hat, shoes, and stockings for the beadle. We find and present that the sum of 2£8s7d sterling be laid in on the inhabitants of the said borough, and when collected be paid as follows: viz the sum of 1£7s5d to Mr Jonathan Gregg, being so much expended by him in providing carriages for Colonel Sydney's regiment of dragoons. (p 102).
1721-1726 - He served as a member of the Ennis Corporation's Grand Jury. (p 409).
1752-On 24 Jun, A presentment to buy a new book for the borough, We find and present that the sum of 6£16s6d sterling be laid in on the inhabitants as usual and when collected to be paid to Mr Jonathan Gregg, to buy a new book for the use of the borough ( the old book being in a tattered condition), wherein all the bylaws and presentment, together with the names of the freemen of said borough, as in said old book, shall be transcribed by said Mr. Jonathan Gregg, to which said jury do sign for said presentment of money. (p155-6).
1755 - On 13 Oct, presentment #339, made in favor of Mr Jonathan Gregg, Deputy Provost of said borough, We find and present that the sum of 5£ sterling be laid in on the borough as usual, and when collected to be paid by the treasurer of said borough to Jonathan Gregg, Gentleman, for good services to be done in the said borough, provided said sum be not collected before 24th June next. (p185).
(pp 399-400) List of Provost, Deputy Provosts, and Burgesses:
29 Sep 1751 Deputy Provost Jonathan Gregg
30 Sep 1754 " " " "
29 Sep 1755 " " " "
29 Sep 1756 " " " "
29 Sep 1757 " " " "
29 Sep 1758 " " " "
29 Sep 1759 " " " "
29 Sep 1760 " " " "
1761 - Presentment made 29 Sep 1761, We find and present that the sum opf 5£ sterling be levied of the inhabitants as usual, and when collected to be paid the treasurer and by him paid over to Jonathan Gregg, late deputy provost, for his long services. (p194-5).

Memorials of Deeds
Sept. 22, 1749 Reg# 9224: Gregg and wife to Robnett
Jonathan GREGG of the city of Dublin, gent., and Ann Gregg otherwise Robnett his wife in consideration of two hundred pounds did release unto John Robnett of the city of Dublin, gent. eldest son and heir of Robert Robnett late of Milltowne in the county of Wicklow, gent . an annunity of thirty pounds a year payable to the said Anne out of lands of Knockrahine in the County of Wicklow by virtue of an article from said John Robnett therein recited and all arrears therof and all Dower and all arrears thereof or demand which the said Ann can claim out of the said land of Knockrahine or out of the lands of Tubbervillerin ye said county or any other estate or inheritance or freehold whereof the said Robert Robnett died seized of which said indenture and the memorial are witnessed by Benjamin Johnston Pulic notary in the city of Dublin and Richard Neville his clerk.

Freeman Journal, Dublin, Ireland, Saturday, May 5, 1764:
DEATHS. A few Days ago, at Inch in the County of Clare, Mr. Jonathan Gregg, aged 95.