For the first time ever, DART services in Dublin now run every ten minutes during weekdays.
The improved frequency will apply from just before seven in the morning until eight in the evening.
Irish Rail spokesperson Jane Creegan says the changes mean that journey times could be a little longer.
"To get through the city centre, we now have more trains so it's going to take longer to get them through with the stopping patterns".
She explained that the journey time would slow down by about "two to three minutes" and that "other services remain unchanged or some may be faster".
Frequency will improve to every 10 minutes in both directions between 06:50hrs-20:00hrs Monday to Friday between Bray and Howth Junction, and then alternate between Howth and Malahide.
Services to and from Greystones will still only run every 30 minutes.
The 10 minute DART schedule has been proposed for a long time, but had faced delays during a row with drivers over rosters and training.