A Dublin criminal gang left a pipe bomb near a busy commuter line at Dun Laoghaire.
Gardai believe the device was stashed near the main rail line for later collection by members of the gang.
Although the device was discovered just feet from the rail line it was not intended to be detonated at the site.
The device, hidden in a duffel bag, contained explosives and a detonator, but was not armed.
It was discovered near a busy pedestrian railway bridge in Dun Laoghaire.
A source told the Herald: "It appears that local criminals were using the rail line as a hiding spot, where they would drop off and collect contraband and, in this case, a pipe bomb. The device was not targeted at the rail line or rail users."
Thousands of DART and mainline rail passengers had their travel plans disrupted.
The bridge close to where the bomb was discovered is a busy walkway and is regularly used by families to access Dun Laoghaire's west pier, a popular spot for walkers.
A member of the public spotted the crude weapon from the bridge at around 12.15pm yesterday and immediately alerted gardai, who contacted Iarnrod Eireann and called the Defence Forces Explosive Ordnance Disposal team.
The scene was declared safe at around 2.50pm and the device was taken to a secure military location before it was handed over to gardai.