Dunlop, 32, was taking part in practice for the Skerries 100 road races in County Dublin when the crash happened.
He was a member of the renowned Dunlop family and was a winner at the North West 200 and Ulster GP.
William’s father Robert also tragically died as a result of an accident at the North West 200 in 2008, while his uncle Joey was killed in a crash in Estonia in 2000.
Joey holds the record for the most Isle of Man TT wins with 26, while William’s brother Michael has registered 18 wins at the event over the Mountain Course.
A statement released by the Loughshinny Club, the organisers of the Skerries 100, said they “deeply regretted William Dunlop has passed away following injuries received in a tragic accident that occurred during practice which took place today.”
“The Loughshinny Motorcycle Supporters Club extends their heartfelt sympathy to William Dunlop’s family and friends. Our thoughts and prayers are with them at this very sad time.”