Noel Large was a cold-blooded gunman for a Protestant paramilitary group during “The Troubles,” a period of bombings, shootings, and political turbulence that rocked Northern Ireland in the 20th century. Today, he’s a reconciliation activist, working alongside Catholics to keep the peace. Although the situation is more stable today, Catholic and Protestant communities remain divided in cities across Northern Ireland by physical barriers known as “Peace Walls.” On the centenary of Northern Ireland’s birth, World Affairs looks at the lines and borders that define it.