Iarnród Éireann says the underwater inspection of the viaduct that collapsed in Malahide was carried out in line with regulations three years ago.
Spokesperson Barry Kenny said underwater inspections are carried out every six years in line with Railway Safety Commission recommendations.
Last Friday, a 20-metre section of the viaduct collapsed into the water shortly after a passing train driver had noticed subsidence around the rail line.
Iarnród Éireann confirmed that concern was expressed by a member of the public on Monday 17 August about erosion on the viaduct.
Mr Kenny said an engineer carried out an inspection the following day on the basis of that concern, and found the markings were cosmetic rather than structural.
Mr Kenny also said the individual who contacted the company referred to markings on the pier but did not refer to a change in the water flow, as was reported in today's Irish Times.
