Irish Rail says rail services between Dalkey and Greystones have been suspended until further notice.
The line has been closed following a fire at a signalling equipment room in Bray at around 11.30pm last night.
The fire is not believed to have been the result of vandalism or criminal damage, Irish Rail confirmed.
DARTs are suspended between Dalkey and Greystones for the weekend 'at least', with Dublin Bus accepting rail tickets.
Passengers are being advised to transfer to the bus at Dun Laoghaire.
DART services between Dalkey and Howth / Malahide are running, with some minor delays possible.
Irish Rail has apologised for any inconvenience caused, saying: "Signalling teams are assessing the damage and repairs required, but have indicated that services will remain suspended at least for the remainder of the weekend (Saturday 19th and Sunday 20th May).