With the huge over supply of rented property out there, your apartment will be just another number to the estate agent. More than likely they will have a lot more properties on there books belonging to friends, maybe themselves, which will be ahead of you in the queue for tenents.
Atleast by doing it yourselve you know exactly the demand for the property and you can sell it better to potential tenents. I have used Daft and found it very good but if its otherside Dublin made you would be better off advertising it in the local papers, shops etc.