Although prototype diesel locomotives ran in Britain before World War II, the railways of both the Republic and Northern Ireland changed over much more rapidly from steam to diesel traction than those in Britain, due to the island's limited coal reserves and (in the Republic) an ageing steam locomotive fleet.

Northern Ireland operated several diesel shunters as early as the 1930s. CIÉ's first diesels consisted of five shunters built by CIÉ in 1947/48. The initial two diesel mainline locomotives were also built in Inchicore, in 1950/51, and fitted with Sulzer engines and MV traction equipment. This was followed in the mid-1950s with a large order from Britain fitted with Crossley engines, with notably poor results. From the early 1960s, locomotives with more reliable engines from General Motors Electro-Motive Division were adopted. In the late 1960s the Crossley engines were replaced by EMD 645 units in a major re-enginging programme. Since the early 1960s all new locomotives on the two Irish rail systems have been purchased from EMD, with the exception of three from Hunslet Engine Company of Leeds, England, for NIR in 1970.

In 1948, CIÉ commissioned a report from Sir James Milne (General Manager of the British Great Western Railway) on the problems of upgrading its rail services. Despite his recommendation to continue with steam traction, CIÉ decided to push for the complete dieselisation of its railway system, expecting substantial economies of scale.

The first of the two mainline diesel locomotives built by CIÉ at Inchicore Works, each with a Sulzer engine built by Vickers Armstrong, Class 113, entered service in April 1950, following completion of trials.

Subsequently, CIÉ placed one of the largest orders of its time, for 94 locomotives (60 Metro-Vick Class 001, or A Class, and 34 201 Class, or C Class locomotives) with a consortium of four British companies:

Delivered to Inchicore Works between 1955 and 1958, the Class 001 was to become the mainstay of mainline passenger and freight services on the network for the next forty years. Parts were also ordered for a further 19 locos to be built at Inchicore Works.

From the outset, the two-stroke Crossley engines proved under-powered and unreliable. The 001s were not capable of reliably handling mainline work, while the Class 201, no longer required on branch line work due to Todd Andrews's large scale closures, were unsuited to mainline work because of their low power (c.600 hp).

From the early 60s, CIÉ looked to GM, with their history of reliability. 15 121 Class were purchased in 1961. These were modified shunting locos ("switchers") and proved an instant success. One drawback was the single cab construction requiring the loco to be turned at the end of each journey as the drivers refused to operate them "long hood" forward. They were modified in the mid-1970s for multiple unit operation and operated in consist, "long hood" to "long hood", with their cabs at each end. 37 141 Class, delivered in 1962, were fitted with two cabs to overcome this problem. The delivery of these engines brought an end to regular steam working in April 1963. A third batch of GM locomotives (181 Class) were delivered in 1966, which were essentially 141s with more powerful engines.