Coastal erosion could put the future of the existing tunnel on the railway line between Bray and Greystones in doubt by 2035 unless urgent action is taken, an internal report by Iarnrod Eireann has warned.
The company says investment in excess of €100m will be required to implement measures to safeguard key sections of the Dublin-Rosslare line, including the Bray Head tunnel.
An internal report on the issue of coastal erosion said: "Drastic consequences will result if action is not taken immediately. Urgent strategy planning is required with rapid intervention of solution measures essential."
The Iarnrod Eireann board was informed last October that the tunnel now only has a life expectancy of 15 more years in a 'do-nothing' scenario.
The Bray Head tunnel, also known as No 4 tunnel, is the longest in a series of tunnels on the single-track line between Bray and Greystones, at just under one kilometre.
The tunnel itself was constructed in 1917 as a result of the need to move the track inland due to coastal erosion.
The report said the Dublin-Rosslare line had two key vulnerable sections, including a 60km stretch between Dublin and Wicklow. The other is a 17km section between Wexford and Rosslare Europort.
It warned that the frequency of bridge collapses, the washing away of embankments and seawater spilling on to sections of track have been increasing and will continue to do so in the years ahead due to higher waves and more frequent storms.
Iarnrod Eireann chartered engineer Aidan Bermingham said there were more instances of such events in the past 20 years than in the previous 100.
Based on coastal erosion forecasts by the OPW, Mr Bermingham said the main tunnel, which is around 45 metres from the sea, would be lost by 2050.
An Iarnrod Eireann spokesperson said a large number of ways of protecting the Bray Head Tunnel were under consideration but moving it was "the last option".
The options include the construction of a man-made offshore island to dissipate the energy of the sea and a breakwater or "beach nourishment", which involves adding sand to the coastline.
It is understood the company hopes to apply for planning permission for any remedial action required in early 2021.
The Bray-Greystones route, which first opened in 1855, included tunnels and viaducts designed by British engineer Isambard Kingdom Brunel.
However, coastal erosion over the decades meant the line had to be moved inland on a number of occasions, including the requirement for construction of several new tunnels.
The high maintenance costs due to rockfalls, landslides and erosion eliminated all the original viaducts and cliff sections, which led to it becoming known as 'Brunel's Folly'.