Rail track movements caused by construction work on the Dublin Port Tunnel happened on five previous occasions before the alert that shut down rush-hour train services yesterday morning.
The earlier movements, all of which were recorded in the past two months, registered as "amber alerts" which were less serious than yesterday's "red alert" situation.
Irish Rail and Dublin Port Tunnel engineers were last night continuing to examine yesterday's incident, which caused one of two neighbouring tracks to sink by 5.9 mm (a quarter of an inch).
They are linking it to the installation of a 54-metre long pipe which is being pushed through the ground five metres beneath the railway where the tracks run through Fairview Park on the city's northside.
Installation work on the giant pipe, which is the full height and width of the tunnel, began last Wednesday morning.
The installation of an earlier parallel pipe was completed last Saturday. It was during work on the first pipe that the five amber alerts registered.
Monitors attached to the tracks register amber alerts when upward or downward movements of up to 4.5 mm are recorded. With two of the five amber alerts, Irish Rail engineers carried out reinstatement works.
Services were not halted on any of the five occasions.
Irish Rail spokesman Mr Barry Kenny said railway engineers carried out a thorough inspection after yesterday's alert and concluded no structural works were necessary.
"This was a tiny movement of one track relative to another. Both tracks were inspected and both were found to be safe," he said.
The alert sounded at 6.55 a.m. and within minutes all trains between Connolly Station and Killester were restricted to platform.
Some 10,000 DART and intercity commuters on the north side of Connolly Station were without trains during the hour-long shutdown while thousands more on the southside experienced delays because trains were trapped by the closure.
Passengers at Killester were told buses would be provided to bring them into the city centre and there was anger when they failed to show.
Mr Kenny said there were difficulties securing private coaches at short notice and apologised for the inconvenience to commuters.
Services resumed after an hour but with a five mile per hour speed limit which, Mr Kenny said, was for the protection of inspection staff working on the line.
It was mid-morning before all DART and intercity services on the Dublin-Belfast route were back to normal.
Initially it was announced that the alert was a false alarm caused by faulty monitoring equipment. Port tunnel management confirmed later the equipment was correct and a movement had been detected.
The Marino Residents' Action Group, which campaigned against the tunnel and is currently in dispute over compulsory purchase orders on land beneath homes along the tunnel route, expressed concern at the incident.