The 123 was one of the routes I planned on doing in 2013 but never got around to it. It is one of the more familiar routes for me as I see it every day, and it operates at a high-frequency of every ten minutes. It is a cross-city route that does not go very far onto the northside or southside. From north to south it operates from Marino (with a terminus at the junction of Griffith Avenue/Malahide Road) to Kilnamanagh Road in Drimnagh via Ballybough, City Centre, Rialto/Inchicore and Drimnagh. It is the only bus route that operates through St. James's Hospital and on both sides of the Liffey it winds its way through housing estates built predominately in the 1930s.
The route is operated by Summerhill Garage on the day I travelled on EV 43, though some VGs were also noted on the route, including debranded-Airlink VG 46.
On a side note, route 123 actually passes through the city postcodes Dublin 1, Dublin 2 and Dublin 3.