On 26th May 1888, a small group of men met in Tullamore - possibly in the rooms of the Trade and Labour Hall on Crowe Street to establish a GAA Club. It may be that the men who founded the Tullamore Club did so because no Tullamore team had entered the first Offaly championships that had been played off in March and April of 1888. With the championship over they embarked in the following months on practice sessions amongst themselves (at a field off Church Road). The club concentrated on football initially and entered the 1889 Offaly Championship, reaching the semi final stage before losing to Banagher. The following year Tullamore enjoyed revenge as they defeated Banagher to win its first ever Offaly Championship and they retained the title the following year beating Rhode. 

In 1909, Tullamore senior hurlers defeated Clara and Killoughey to reach the county final against Drumcullen. An epic match ended with Tullamore winning 1-9 to 1-8. 

Having played matches at rented lands on Church Road and also at Tinnycross, the club acquired Ballyduff Park at an annual rent of £12. 

The years 1914 - 1939 were hugely successful ones for Tullamore GAA Club. The club won a host of championships and developed its facilities. 

In 1917 inspired by the 1916 rising, the club changed its colours from red jerseys with a green sash to green, white and gold jerseys. That same year, the senior footballers defeated Ferbane to take the senior football title. 

Having being regraded to junior hurling in 1919, Tullamore reached three consecutive junior hurling finals finally winning in 1925. 

The establishment of underage competitions in 1928 brought immediate joy to Tullamore with the minor football title annexed in the first year of its running. Underage success marked theforerunner of the greatest period of success in the clubs history between 1930 and 1935 - Tullamore GAA Club won ar least one title in every grade in both football and hurling. 

The first double of the clubs history was secured in 1932 with the senior hurlers defeating Drumcullen and the senior footballers defeating Rhode in their respective finals. The senior hurlers enjoyed much success in the following years - In 1936 they retained the senior hurling title by defeating Rahan and then in 1937 they completed the four in a row. 

Tullamore GAA Club set about developing a piece of land on the Arden Road in 1932 and on the 24th June 1934 O'Connor Park was opened. 

In the early 1920's Tullamore stopped wearing the green, white and gold jersey s amd had instead given the right to wear them to the Offaly county team. At that point, Tullamore teams took to the field wearing plain green jerseys. Now at the end of the 1930's, further change came as Tullamore adopted a royal blue jersey with white collars and white cuffs. The Greens were now the blues and this was how they were to stay!

Four Offaly minor hurling titles were added in five years - 1941, 1943, 1944 and 1945.

The 1950's opened on a bright note with a minor football and hurling double secured as well as the under 16 football title. After 1950, Tullamore won seven under 16 footabll championships in none years and there was underage success in hurling too with the under 16 championship won once and the minor championship won twice. The intermediate football title was won in 1953 for the first time in the club's history. Tullamore won the senior football championship in 1954 and were for the first time presented with the Dowling Cup. The return of the Dowling Cup in 1956 came on the sixtieth anniversary of the clubs first senior football championship success (1896). In between the two football successes the senior hurlers enjoyed success as they claimed the 1955 championship over rivals Drumcullen in the final. They reclaimed the title in 1959 and the final success of this era saw the senior footballers claim the clubs 21st  senior football championship in 1963. 

During the 1950's and 1960's the facilities at O'Connor Park were extended with the construction of new dressing rooms. 

The highpoint of the 1960's saw the senior hurlers beat St. Rynagh's to take the senior hurling title in 1963. Two under 16 football titles were won in 1966 and 1967, as was a minor football title in 1967 and a minor hurling title in 1966. After the minor championship success in 1966, no Tullamore team won an underage hurling title until the under 14's won a B championship in 1982. A Minor B championship was won the following year in 1983. 

The club won its first under 21 football title in 1969 with victory over Eire Óg followed by senior success in 1973. In 1977 Tullamore club enjoyed the most successful year in its history with senior, under 21 and minor football titles won. 

The club's minor footballers won the championship in 1982 and 1986 and the under 14's and under 21's won the championship in 1989. There was also success for the intermediate and minor b hurling teams as both claimed championship titles. The 1980's also saw the development of O'Brien Park as land was purchased from the Midland Health Board. 

The under 12 footballers won four in a row between 1997 and 2001, the under 14 footballers won three in a row between 1999 and 2001, the under 16 footballers won three championships between 1997 and 2003 and the minor footballers won four championships between 1999 and 2005. Local success provided the platform for national success. The under 14s won Division 3 of the Féile All-Ireland in 1999 and came back the following year in 2000 and won Division 2. The second intermediate title came to Tullamore in 1999. The 2000 Offaly senior football championship was an extraordinary affair. It began in April and did not finish until December. Tullamore recorded a two point victory over neighbours Shamrocks in the final. A 23 year wait for a senior football championship had come to an end. Senior success followed in 2002 for the senior footballers with victory over Gracefield on this occasion. 

In 2001 an Extraordinary General Meeting of the club was arranged to decide on the proposal to lease O'Connor Park to the Offaly County Board. The proposal was carried by a large majority. This led to the development of O'Brien Park as facilities were further enhanced by the erection of a stand, railings, hurling wall, ball catchers and dugouts. Vitally both pitches were also floodlit to premium standard allowing for their usage all year round. 

The efforts made in hurling began to reap dividends with an Under 12 A championship won in 2004 and Under 14 A championship won in 2006. 2007 brought senior football success and then in 2009 the senior hurling team created a huge shock by reaching the 2009 senior hurling final. In the final they met Kilcormac Killoughey and were rank outsiders. On a wonderful afternoon in O'Connor Park, Tullamore played with style and skill and conviction in running out clear winners. In 2012 the senior footballers came up against Rhode in the senior football final who if victorious would draw level with Tullamore on 26 titles each. It was however to be Tullamore's day and the Junior A footballers a week later followed the senior football championship with a title of their own. ​