You might want to try the Luas site, it used to have a fare calculator but its been missing its a challenge even to find the station list
There is a internal power struggle in IE as to who owns the website so thats why there are 2,
www.dart.ie jsut points to a page on the second site
The error message on the IE site is fairly reasonable, in order to proceed at least one reservable leg of your journey must be selected. If you try to reserve a DART journey you should get a error dialog stating bookings not possible for that combination of stations, if you hit cancel and try again it lets you move to the select train page
Why bother we have both online
http://www.platform11.org/passenger_info/