Hi Evelyn
I'm having problems finding some totally reliable information about Mary Stewart and her Preston husband and what I have found is contradictory but I reasonably sure that the Jane Preston who married Alexander Breckenridge is definitely not the daughter of Mary Stewart and Phineas, aka Archibald Preston. Wikipedia are incorrect on this point and don't actually cite a source for Jane Preston.
The sources I have found include The Complete Peerage, 2nd ed, vol. 6, p. 56, which states that George Forbes, 3rd Earl of Granard, married "in 1709, Mary, widow of Phineas Preston of Ardsallagh, co. Meath ... she d. 4 and was bur. 11 Oct 1758, aged 85" which would place her birth in about 1673.
However Burke's A genealogical and heraldic history of the landed gentry of Ireland, p. 576 under Preston of Swainston, states that Mary Stewart, married John Preston of Ardsallagh, who was born in 1672 and died 1702-03, having had issue a son Phineas, who died without issue (d.s.p.) and a daughter Mary, who married Peter Ludlow.
The Complete Peerage, 2nd ed., vol. 8, p. 263 under Ludlow, has Peter Ludlow, of Ardsallagh, co. Meath, son and heir of Peter Ludlow, of Ardsallagh by Mary daughter and heir of John Preston of Ardsallagh but without mentioned the name of Mary's mother's.
Lastly The Peerage of Ireland, 2nd ed., vol 3 by John Lodge, edited by Mervyn Archdall, on p. 80 has that John Preston of Ardsallagh, married while still a minor, Mary Stewart and had a son Phineas, who died while still a minor aged 20, and Mary, heir to her brother, who married Peter Ludlow. However this work confuses John Preston of Ardsallagh, with another John Preston who served in the Irish Parliament for Meath in 1731.
Based on all the above I think the chronology makes it more likely that Mary Stewart did marry John Preston of Ardsallagh who died in 1702/03 and had two children; a son Phineas who may have initially succeeded his father, but died while still a young man, and his sister Mary was his heir, which means he can't have been the father of the Jane Preston (Breckendridge).