Hi BrixnMortar, the first thing to do is register the property with failte Ireland. You may be too late to appear in this years magazine however you can be listed on their website and leave promotional material in local tourist offices etc. Registering also gives you a star rating and gives an idea of how much you should be charging for your property as well as bringing you into line with health and safety requirements eg. smoke alarm, fire blanket etc.
Depending on your property's location and near by amenities you could advertise in specialist magazines yourself, though this can be costly. Agencies typically work on a 20% to 25% commission so this can be quite a chunk of income also.
Some of the rental agencies prominent in Ireland at the moment are irishvacationrental.com lodgings-ireland.com and rentacottage.ie
I have been renting self-catering cottages in the north west for 28 years and findthe most cost effective way of having your property occupied on a regular basis is to offer an outstanding product and service and work on repeat custom and word of mouth.