I immediately found contradictions in the available information. The parliamentary records indicate that William D (of Kilaghy Castle and the MP for Thomastown and Bantry), who married Frances Green, had a father also called William D who married Elizabeth Armstrong, daughter of Philip Armstrong – whereas the diaries indicate that he was the son of the William D who married Elizabeth Croasdaile. In looking at the published Armstrong trees it seemed that there was only one recorded Philip Armstrong who might fit, and that was the Philip who was born in 1641. He joined his cousin Sir Thomas Armstrong (Monmouth’s close associate, who was an active promoter of the Rye House plot), in King Charles’ Horseguards, but is stated to have died unmarried in 1711. This Philip had a brother William (married to Alice Coghlan) who had a son Philip Armstrong who married Elisabeth Judge of King’s county, and their daughter Elizabeth married (circa 1730) William D of Coolrain, Queens County. Hence I suspected that the Parliamentary records on this were wrong.
However, later I found in the 1905 ‘Notes and Queries’ an article detailing the capture and detention in 1655 by parliamentary forces of Sir Thomas Armstrong. After three years of prison in the Gatehouse he was removed to the Tower of London and was then sent on to Jersey where he was held for six more months. He was discharged on 30 June 1659. The sureties for his bond were Philip Armstrong and Matthew Clifford (presumably Master of the Charterhouse, whose patron was George Villiers, second Duke of Buckingham), both of Westminster, who posted £1,000 ‘that Sir Thomas shall do nothing prejudicial to the peace of the Commonwealth’. The dates mean that this is unlikely to have been the unmarried Philip Armstrong who was born in 1641. At the Restoration, King Charles returned Sir Thomas to his military posts in Ireland and granted him the rights for copper coining in Ireland. (He died in Dublin in 1662). I don’t know for sure what Philip Armstrong’s relationship to Sir Thomas was, but I assume a brother. The dates certainly make it possible that a daughter of this Philip Armstrong was the Elizabeth Armstrong, referred to in the Parliamentary records as the wife of William D the MP’s father (whereas the dates don’t comfortably work for the first, and he died unmarried). If William Despard and Elizabeth Armstrong married in say 1670 then they were probably born circa 1650.