Ratlines / Stuart Neville.

Enhanced descriptions from Syndetics:

Ireland 1963. As the Irish people prepare to welcome President John F. Kennedy to the land of his ancestors, a German national is murdered in a seaside guesthouse. Lieutenant Albert Ryan, Directorate of Intelligence, is ordered to investigate. The German is the third foreigner to die within a few days, and Minister for Justice Charles Haughey wants the killing to end lest a shameful secret be exposed: the dead men were all Nazis granted asylum by the Irish government in the years following World War II.

A note from the killers is found on the dead German's corpse, addressed to Colonel Otto Skorzeny, Hitler's favorite commando, once called the most dangerous man in Europe. The note simply says: "We are coming for you."

As Albert Ryan digs deeper into the case he discovers a network of former Nazis and collaborators, all presided over by Skorzeny from his country estate outside Dublin. When Ryan closes in on the killers, his loyalty is torn between country and conscience. Why must he protect the very people he fought against twenty years before? Ryan learns that Skorzeny might be a dangerous ally, but he is a deadly enemy.