From "MappingDubliners.org", this site shows a video with maps of the route the race took in "After the Race" along with images and descriptions of the various areas they passed through. The link can be found HERE.
From "Grace's Guide to British Industrial History", this site shows the details of the 1903 Gordon Bennett Cup race which is the most likely inspiration for the race referred to in "After the Race". The link can be found HERE.