Anyone with information is urged to come forward.
Gardaí have said they are investigating all the circumstances surrounding the discovery of a body in unexplained circumstances on the N3, at the Snugborough Road Bridge.
At 3.30am on Monday morning, Gardaí attended the scene following reports of a male lying unconscious in the area.
The man, aged in his 40s, was brought to James Connolly Memorial Hospital where he was later pronounced dead.
Gardaí are appealing for any witnesses or for anyone with any information to come forward.
Gardaí are also appealing for any road users who may have camera footage (including dash-cam) who were on the Snugboroguh 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 on 01 666 7000, the Garda Confidential Line on 1800 666 111 or any Garda Station.
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