Deenihan had much success at club and divisional level with his local teams. At under-age level he won an under-14 North Kerry Championship with Finuge in 1963. Five years later in 1968, he won an under-16 North Kerry Championship with the Listowel Emmet's club. He also played with the St Senan's club at minor level. With Finuge Deenihan also won a North Kerry Senior League title in 1970, as well as a North Kerry Senior Championship title in 1987. With his local divisional team, Feale Rangers, he won two senior county championship medals in 1978 and 1980. While in college at St Mary's University, Twickenham in London he played with the Kingdom club and won a London Senior Football Championship.
Inter-county
By the late 1960s, Deenihan was a key part of Kerry's inter-county set-up and was playing on the county's minor team. In 1970, he won a Munster minor medal, however, he never won an All-Ireland medal. Deenihan later moved on to the county's under-21 team, where he won back-to-back Munster titles in 1972 and 1973, as well as an All-Ireland title in 1973.
By this stage, Deenihan was also a member of the Kerry senior football team. He won his first National Football League medal in 1973, however, Cork were the kingpins in the Munster Championship. Two years later in 1975, Deenihan captured his first Munster Championship and All-Ireland medals at senior level, under the new management of the legendary Mick O'Dwyer. It was the beginning of a glorious era for Kerry football and Deenihan played a key role in orchestrating much of the success.
In 1976, Deenihan won his second Munster title, however, Dublin gained revenge for the previous year by defeating Kerry in the All-Ireland final. 1977 was another frustrating year as Kerry won another set of National league and Munster titles but lost out to Dublin yet again, this time in the All-Ireland semi-final. By 1978, Kerry were ready for success. They easily won another Munster title and breezed into the All-Ireland final to face Dublin. The game itself is remembered for Mikey Sheehy's cheeky goal which he scored by lobbing the ball over the head of Paddy Cullen. The final score was 5–11 to 0–9 and Deenihan had finally secured a second All-Ireland medal. In 1979, Deenihan captured a fifth provincial title before later capturing a third All-Ireland medal following another emphatic victory over Dublin.
In 1980, Kerry breezed through the Munster Championship once again and later defeated Roscommon to win a third consecutive All-Ireland title. In 1981, Deenihan was appointed captain of the Kerry senior football team. It was a very special year as he collected his seventh consecutive Munster Championship medal before later leading Kerry out in a fourth All-Ireland final appearance. On the day the men from 'the Kingdom' easily defeated Offaly to give Deenihan his fifth All-Ireland medal. He rounded off the year by being presented with an All-Star award. At the start of 1982 Deenihan won a third National League medal when he captained Kerry to victory over Cork. Later that summer he broke his leg in training. He remained out of the game for eight months, meaning the injury effectively ended his inter-county career. Deenihan's last appearance in a Kerry jersey was in May 1983 when he played a challenge game at the opening of the Tarbert GAA field.
Deenihan also won Railway Cup medals with Munster in 1975, 1976, 1978, 1981 and 1982.
In May 2011, he set out proposals to acquire from the Bank of Ireland the old Irish Parliament building in College Green for the state, to use as a cultural venue.[10]
In November 2011, he met with the stars of Titanic: Blood & Steel, while they were filming on location in Dublin.[11]
He was reported by the Irish Examiner as having announced the awarding of the title of City of Culture 2014 to Limerick in June 2012 without indicating the criteria for selection.[12]