In Dublin, the story of an I.R.A. insurgent-turned-writer leads to historic watering holes where the city’s oldest whiskeys flow. The man who first declared himself “a drinker with a writing problem” understood what distilled spirits mean to a culture’s identity; but it was his understanding of how to overcome a violent past that makes his words especially relevant. Meanwhile, in Belfast, a small but daring group of crime novelists are using timeless pubs and pints of Guinness to confront decades of real-life killings in their city.