32-year-old Dunlop was making his return to racing at the national Skerries 100 event having chose to sit out the 2018 Isle of Man TT.
A statement released by the the Skerries 100 organisers, said they "deeply regret" to announce he "passed away following injuries received in a tragic accident".
"I would ask the entire community, not just the constituency's road racing fraternity, to remember the Dunlop family circle in their thoughts and prayers as they face the challenge of yet another road racing tragedy".
His brother Robert was also a successful road racer, and Robert's two sons - William and Michael - have carried on the mantle.
Dunlop recently revealed he was expecting a second child with his partner Janine.
You don't need a deep knowledge of motorcycle racing to know the Dunlop family. Soon after, William also walked away at the last minute from the recent Isle of Man TT, expressing a desire to spend more time at home.
It is believed that William Dunlop was riding an R1 Yamaha bike when he crashed on part of the circuit known as Sam's Tunnel at approximately 5pm on Saturday.
Not only did William clinch countless wins at the smaller road races, he was also able to make the step up at worldwide level, like Joey and Robert before him and just as his younger brother Michael has done over the past 10 years.
"Everyone's thoughts are with the family".
William achieved four wins at the North West 200, including successes in the 125cc and 250cc races in 2009, a Supersport triumph in 2012 and a Superbike victory in 2014.
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