Record numbers will take part in the Dublin Marathon tomorrow but people travelling from the southside will be affected by Dart closures.
Irish Rail said that major track works will mean no Dart services operating between Connolly Station and Dun Laoghaire today and tomorrow.
"Obviously people who are travelling from the southside will need to use alternative means," spokesperson Barry Kenny told the Herald.
Speaking about other routes from Howth, Drogheda and Maynooth, he said: "We will have additional early morning services to allow people who are participating in the marathon to travel in, so we will have some extra trains on each of those routes.
"We are doing a series of works, repairing the roof of Pearse Station and we are also doing track works."
Mr Kenny defended the decision to carry out the works this weekend, saying: "We don't have works like this that impact weekdays. We don't affect weekday commuting."
He said that the works being done on a Saturday and Sunday won't affect the two biggest travel days of the bank holiday, which are Friday and Monday.
"It isn't the first year we have had works that have coincided with the marathon, and the marathon has operated successfully," he said.
"Ultimately the scale of these works is such that it does require the 48-hour suspension, and if you were to do that on a normal weekend, you are going to hit that Sunday evening 'mass return' as it were for people travelling."
He added that the stoppage had been communicated extensively in recent weeks and said that Dublin Bus will accept rail tickets for affected areas.
Road closures will be in place across Dublin city and county during the marathon, with around 22,500 participants set to hit the streets.
The course will start at Fitzwilliam Street Upper at 8:55am and will finish in Merrion Square North up to 4.45pm.
Organisers say that the event is worth €25.6m to the city, and it's the fourth largest marathon in Europe.
Some 1,486 volunteers will be on hand to make sure that the event runs smoothly.
Race organiser Jim Aughney said: "We encourage runners in the KBC Dublin Marathon to get drinking early in the run, that's really critical. It's truly not a good idea to wait until you're thirsty. We have 10 water stations set up along the route."
More than 60,000 litres are expected to be consumed by 22,000 runners.
Ishka Irish Spring Water said that eight trucks carried the water on 74 pallets from Limerick to Dublin for the event, and all bottles will be collected for recycling.
It's the 40th anniversary of the marathon, with international elite runners set to compete against top Irish athletes.
For others taking part, it's all about the achievement of crossing the finishing line.
A total of 13 runners will have taken part in all the races since it began in 1979.