O'Kearney turned to ultrarunning at the end of a successful career as a marathon runner which saw him win the 2005 Tresco marathon. A few weeks after running his last marathon in Dublin in 2006, he made his ultramarathon debut in Central Park in New York. And what a thrilling debut it turned out to be, with O'Kearney in a breakaway group of four shortly after the start, and pulling clear on his own at half way to go farther and farther ahead to win the race by over 6 minutes in an amazing display of front running.
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| Over the next year, O'Kearney enjoyed the ultrarunning equivalent of Stephen Roche's annus mirabilis in cycling (which saw him claim the Tour De France, Giro D'Italia and World Championship in one year). A few weeks after becoming the first Irishman to win the New York ultra marathon, O'Kearney made a winning debut in a new course record over the shortest ultra distance, 50K, in Schinnen, in the fastest time ever recorded by an Irishman in an IAU-sanctioned 50K race. |  |
| A few weeks later, he ran the Connemara ultramarathon as a training run as part of his buildup for his international 100K debut. O'Kearney's easy long run pace proved competitive enough for third place in the race. A few weeks later, he made his international debut, representing Ireland in the Anglo Celtic Plate 100K in Edinburgh. Despite being hampered by both injury and illness, he finished a creditable 8th, and claimed a silver medal in the Irish championships as the second Irishman home. |  |
| After a four week layoff to recover from illness and injury, O'Kearney was drafted as a late injury replacement to the Irish team for the World 24 Hour championships in Quebec. With less than 2 weeks to prepare, nobody could have expected O'Kearney to deliver the greatest ever performance by a debutante but that's exactly what he did. In 35 degrees heat and 90% humidity, O'Kearney ran 205 kilometres to finish 32nd, ahead of his more experienced teammates that included Tony Mangan, Eoin Keith, Richard Donovan and Eddie Gallen. The effort left him in hospital overnight and in a wheelchair for weeks and crowned him as Irish 24 Hour National champion. |  |
| A few months later he returned to New York to finish on the podium in Central Park once more in the prestigious New York ultra marathon, further emphasizing his versatility and ability to perform over the full range of ultra marathon distances. |  |
| In early 2008, he made a winning debut over 6 hours in Brno in a new course record, establishing new national 6 hour records for all surfaces in the process, and a new 50K indoor record. |  |
Injury hampered him on his second attempt over the 24 hour distance at the 2008 world championships in Seoul where he finished just inside the top 100 (and second Irishman). He continued to struggle with injuries into 2009 and was forced to abandon in the 2009 World championships in Italy after 6 hours. Immediately afterwards, he announced his retirement from the Irish team.
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| Although his career was ultimately cut short by injury, it's unlikely any Irish ultra runner will ever achieve as much in so short a time. His ability to perform over the full range of distances on very short notice was unprecedented. |  |
| As his running career wound down, O'Kearney turned his attentions to a new pursuit: poker, with similar results. Within a few months of learning the game he was crowned European Deepstack champion and quickly established himself as one of Ireland's top professional poker players. |  |
| Throughout his ultra running career, he was coached by the legendary Norrie Williamson, author of "The Lore of Running". His harrowing account of his 24 hour debut, "Long Night's Journey into Day", was published on numerous ultrarunning sites and won worldwide acclaim. |  |
