During the 12-month period up to May 1 this year, the Irishman is said to have earned a whopping $180million, though $150m of that came from the sale of a majority stake in his whiskey brand.
Barcelona and Argentina forward Messi was second on the list and set a record as the highest-earning soccer player after bringing home $130 million while Portugal and Juventus forward Ronaldo earned $120 million to sit third among the top 10 highest-paid athletes.
NFL quarterback Dak Prescott ($107.5 million) of the Dallas Cowboys and four-times NBA champion LeBron James ($96.5 million) rounded out the top five.
Formula 1 world champion Lewis Hamilton was the only Brit to make the top 10 list, coming in eighth, while no female sports stars made the cut.
Forbes said its on-the-field earnings figures include all prize money, salaries and bonuses earned during the 12-month period while off-the-field earnings are an estimate of sponsorship deals, appearance fees and licensing income.
Additional reporting by Reuters.