The issues have arisen since more frequent services were introduced this week
More DART services are to begin running in Dublin from next week, following a number of complaints about the new timetables.
Irish Rail introduced the higher frequency services last Sunday - most notably upgrading to a 10-minute frequency between Bray and Howth Junction during weekdays.
However, many passengers have complained about capacity issues since the change.
To tackle the problem, the company has announced two additional morning services from Portmarnock, as well as an extra service from Clongriffin.
In a statement, the rail company said: "Iarnród Éireann believes that these changes will address the capacity issues that have arisen particularly at inner DART stations such as Harmonstown and Killester and the frequency concerns from customers from Portmarnock and Clongriffin.
"Portmarnock now has the same number of peak morning services as it had before the timetable change and Clongriffin has one extra."
Irish Rail spokesperson Jane Cregan is hopeful the extra services will ease any capacity issues.
She observed: "During the week, it has come to our attention that we do have some capacity issues on the northside of the city.
"From Monday next, we will have a service at 7:29am and 7:42am from Portmarnock, and at 7:45am from Clongriffin."