The body was discovered at the Snugborough Road Bridge in Blanchardstown at around 3.30am on Monday morning.
The man, aged in his 40s, was brought to James Connolly Memorial Hospital where he was later pronounced dead.
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Gardaí are appealing for any witnesses or for anyone with any information to come forward.
They are also appealing for any road users who may have camera footage and who were on the Snugborough Road or travelling on the N3 between 1.00am and 3.30am to make this available to gardaí.
Anyone with any information is asked to contact Blanchardstown Garda station (01 666 7000), the Garda Confidential Line (1800 666 111) or any Garda station.
