When the members of the Irish Republican Army attempt to defend the church building, Devlin picks up a gun and joins them.
He succeeds and is later described by a Scotland Yard detective as "the most cold blooded executioner the movement has seen since Collins and his murder squad".
During an intelligence mission inside the neutral Irish Free State, he is captured after a gunfight with the Garda Síochána, but later escapes from hospital in Dublin.
In the 1975 novel The Eagle Has Landed, Devlin has recovered from his wounds and is teaching Irish language literature at Friedrich Wilhelm University in Berlin.
Devlin, who is hiding in Lisbon and planning to escape to America, is persuaded by SS Gen. Walter Schellenberg, then chief of the Ausland-SD, to rescue Steiner from the Tower of London.
After they return to Germany, Devlin and Steiner prevent Reichsführer-SS Heinrich Himmler from seizing control of the Reich.
The novel ending states that Devlin now lives in a cottage in County Mayo and that he and Steiner remain friends.
In other novels, Devlin has made cameo appearances as a mentor to Sean Dillon (in Drink with the Devil, The President's Daughter and Day of Reckoning) and Martin Brosnan (in Eye of the Storm).