Armstrong served as
Bishop of Cashel and Waterford from 1968 to 1977,
Bishop of Cashel and Ossory from 1977 to 1980. His translation to the See of Armagh in 1980 catapulted him into the fraught world of Northern Irish politics, a deteriorating security situation and the heightened community tensions of the Hunger Strikes and later still, the
Anglo-Irish Agreement. Along with fellow Church of Ireland bishops he regularly met political leaders to offer analysis and informed opinion: government minutes of some of those meetings have now been released. These meetings took place with political leaders in both jurisdictions on the island and Armstrong often led delegations to Dublin for talks. He formed such a warm and effective working relationship with his Armagh neighbour Cardinal
Tomas O'Fiach that when he announced his retirement, it was recorded that his successor
Robin Eames was regarded by comparison as a "cold fish". He retired in February 1986 at the age of 70 and spent his short retirement in Skerries, County Dublin. He died in July 1987. ==References==