On the east side of Dublin
Poolbeg: €280 per metre for a year plus membership; €20 a night for visitors.
Dun Laoghaire: €435 per metre with facilities for a year, €290 per metre without; €3.60 per metre per night for visitors.
Howth: €81 per metre per month; daily rate €3,20 per metre, minimum daily charge €20.
On the west side of Dublin
Grand or Royal Canal: current maximum €278 per year, irrespective of boat length, less than Dun Laoghaire charges per metre (for a berth with no facilities).
Admittedly you can go to more places from Dun Laoghaire (like, er, Holyhead), but on the other hand you can go boating from west Dublin in all but the most extreme conditions and there are more pubs along the way.