In a large saucepan, simmer the Guinness until reduced by 3/4 in volume, about 8 minutes.
Combine the cream, milk, and sugar in a medium, heavy saucepan. Scrape the seeds from the vanilla bean into the pan and add the vanilla bean halves. Bring to a gentle boil over medium heat. Remove from the heat.
Beat the egg yolks in a medium bowl. Whisk 1 cup of the hot cream into the egg yolks. Gradually add the egg mixture in a slow, steady stream, to the hot cream. Cook over medium-low heat, stirring occasionally, until the mixture thickens enough to coat the back of a spoon and reaches 170 degrees F. on an instant-read thermometer, about 5 minutes. Remove from the heat and strain through a fine mesh strainer into a clean container. Cover with plastic wrap, pressing down against the surface to keep a skin from forming. Chill in the refrigerator for 2 hours.
Remove from refrigerator and add the Guinness reduction, whisking until well blended. Pour into the bowl of an ice cream machine and freeze according to the manufacturer's instructions. Transfer to an airtight container and freeze until ready to serve.
Yield: 1 quart