A Dublin Bay cruise on a one-way basis Dún Laoghaire to Howth or vice versa is being introduced by a sister ferry company of Doolin to Aran Ferries.
Start date seems to be this coming Saturday, 13th April, 2013. Details here:
http://www.dublinbaycruises.com/
To get to the rail point of this - concession DART tickets are available to cruise customers. Details here:
http://www.dublinbaycruises.com/booking/
An interesting and innovative service.