I tried to send you an email about this family, but you must have changed your email address.
Information found on the Lyon Family Genealogy forum is as follows, "In 1997 I requested research help from the Cumberland County Historical Assoc. in Pa, who confirmed that Plunket, born June 22, 1765 was son of Capt. William Armstrong, who died in 1804 in Derry Twsp, Mifflin Co., and who was the son of Capt. William Armstrong (brother of Col. John Armstrong) who died in 1785. That researcher throught Plunket was probably married in Mifflin Co, but in 1997 earch church records for the area were not available."
I have been in contact with two of Plunket's descendants doing research on this family.By the way, the records seem to show that Plunket married Margaret Lyon, not Margaret Marten.There is an entry on Ancestry.com that lists a death certificate for Margaret Lyon Armstrong, daughter of Thomas and Margaret Lyon, the date of death matches that on her headstone on findagrave, where she is buried with Plunket.
Please let me know if there is any other information you would like on this family.I am descended from Plunket and Margaret's daughter Rebecca.