The floodmaps.ie website is based on historical flooid events, it isn't a statistical model. Just because the house didn't flood before doesn't mean it won't in the future. It could be that it is outside of the 100 year event (asd in there is a 1% chance of it flooding every year) but inside the 500 year event zone. Also, with development over time the drainage in the area could have changed dramatically so that a flood event will be far more severe.
Are there flood defences in place by the council, levees, diversions etc.? What is the elevation of the house using OS maps (they use Malin Head elevation datum) and the elevation of the river? Can the council provide mapping of the proposed flood area?
Personally, i would stay away from a house that is potentially in an area subjected to flood. If it happens to you, whether insured or not, there is a lot of damage - physical and emotional, and you could possibly result in being homeless for a period of time.