The 121 Class locomotives were manufactured from December 1960 to January 1961 and numbered B121 to B135 inclusive. From the early 1970s onwards several locomotives of this class dropped the "B" prefix from their fleet number when re-liveried. The last two locomotives that survived in traffic (124 and B134) were both withdrawn from service on 3 May 2008. Prior to 1961, almost all Irish diesel locomotives were built in Great Britain, but from the 1960s onwards, GM became the sole supplier of locomotives to CIÉ, which eventually also extended to Northern Ireland Railways locomotives at a later stage. These were EMD's first ever fully American-built locomotives delivered to Europe.