The Class 121 locomotives were a typical American-style single cab ‘road switcher'. The layout of the cab was quite different from the other conventional CIÉ diesel models of the time, with the controls to the side of the driver, rather than the front. Due to apparent driver complaints of reduced visibility when operating with cab trailing, it was ultimately decided that these locomotives should only operate in a cab-leading formation. Later conversion for multiple-unit working allowed two 121 class locomotives to be coupled hood-end to hood-end, removing the need to turn them around for their return journey.