The numbering system in the beginning

The Dublin United Transport Company (DUTC) began operating buses in 1925, introducing their initial 13 routes numbered 39 to 51. The tram lines utilised the lower numbers from 1 to 31. Before numbers symbols were used. The route numbering system followed a radial pattern, commencing from Blanchardstown in the northwest (39), progressing clockwise, and concluding with route 51 in the west (Clondalkin). These routes were established between 1925 and 1927, with additional routes introduced in subsequent years; 1928 (52), 1929 (53) and 1932 (54).

Until 1934, these routes constituted the entirety of the bus network. However, that year, the DUTC assumed control over multiple routes previously operated by private companies. Additionally, new routes were introduced in 1936. The routes taken over in 1934 were not numbered until 1936, when they were assigned numbers from 55 to 84. The subsequent phase involved the replacement of the tram system with bus services, occurring in the following sequence; 1938 (23, 24, 28 and 30), 1939 (11, 12, 16, 17 and 19), 1940 (2, 3, 9, 10, 18, 21 and 25), 1948 (14 and 15) and 1949 (6, 7 and 8).

During the transition from trams to buses, the following additional bus routes were also introduced; 1939 (13, 20 and 38), 1940 (26 and 27) and 1944 (22). In 1945, following the merger of the DUTC and the GSR to form CIÉ bus service 85 transferred to the new company also.

During the 1950s, the remaining gaps in the numbering system were filled in by CIÉ. In 1951, the numbers 1 and 36 were used, followed by number 5 and 35 in 1952 and then number 34 in 1953. In 1958, the takeover of the Great Northern Railway of Ireland (GNR) led to the addition of numbers 29, 31, 32 and 33. The number 86 replaced the Harcourt Street to Bray rail line in the same year, whilst the number 87 and 88 replaced the Hill of Howth tramway in 1959. Number 4 was finally utilised in 1965. Notably, the numbers 57, 71, 73, 74, 75 and 76 were not in use by 1968. Numbers 57, 71 and 73 have not been active since the 1940s.

"A" suffixes were used from the beginning, with route 39A to Clonsilla in 1926. From 1936, many routes with an "A" suffix were not related to the route with same number. Most of these did not survive or were changed within a few years. However, the 44A and the 51A survived. The 44A was an extension of the original 44 in Clontarf. When it replaced the  44, the number 44 was given to the Enniskerry route in 1936. The 44A was merged with the 30 to form the 130 in 1996. The 51A was the service between the City Centre and Beaumont. It never had any connection with the 51 which operated between the City Centre and Clondalkin. The 51A was a relic of the strange numbering that happened in 1936. The number 51A was last utilised in 2009.

"B" suffixes were quite rare in the early days. The 54B was the first use of a "B" suffix in 1939, but this service only lasted a few months. The 50B was the next "B" service and the only one for many years. This was followed by the 41B (1948), 47B (1949), 15B (1952), 44B (1953), 78B (1957) and 42B (1958). The first use of a "C" suffix was the 40C in 1974, this was followed by the 38C, 42C and 78C. Express services appeared in the late 1980s, these services branded as Cityspeed utilised the "X" suffix.

The first instances of the "D" and "E" suffixes were observed in 2000 with the 46D and 46E. Subsequently, in the same year, the Nitelink services transitioned from utilising letters to employing alphanumeric designations, with the "N" suffix. In 2001, the 15F marked the first usage of the "F" suffix.

Higher route numbers began to emerge in the mid-1980s. DART feeder services contributed to this with the introduction of routes 90, 101, 102 and 103 on the northside and 111, 113 and 114 on the southside. In the 1990s, important routes received numbers above 100, these often replaced existing routes. Examples include routes 120, 121, 122, 123, 130, 134 and many more. Initially, Tallaght Localink services used designations T01, T02 and T03 before being changed to 201, 202 and 203. Other minibus services that served the outer suburbs were given numbers above 200.

Several routes could be traced back to their original counterparts with the same numbers. Additionally, routes 122, 130 and 150 can be traced back to the original routes 22, 30 and 50 respectively.

The reason the 17A was allocated to the northside orbital route while it's plain partner 17 was the southside orbital, was the fact that it was planned to connect up both routes to create a semi-circular routing. This oddity has nothing to do with the system created in the 1930's when the plain numbers 44, 45, 50 and 51 operated on the southside while their "A" suffix partners operated northside services. Indeed the 44 and 44A switched sides, at one time the 44 was northside while the 44A was southside.

Tram numbers 1-31 were all used at some stage, although their is some doubt about the 27 which may have been allocated to the City to Drumcondra via Capel St and Dorset St service. This had been an unnumbered tram route which disappeared and then was revived for a period. The revived service may have utilised the number 27. This later became bus service 71 to Walsh Road in Drumcondra.

The National Transport Authority's Bus Connects network redesign project, will see the introduction of alpha affixed spine, orbital, local, peak and express services. There will also be less frequent stand alone services which will compliment spines and serve housing estates. It is intended to use numbers for these services that are currently no in use. So far under this plan we have seen the introduction of the C, H and G-Spines and the Northern, Southern and Western Orbitals.

Iconic service numbers will be discontinued in this plan, some after nearly 100 years in use.