the roads in the Netherlands are as flat as they come (its partially below sea level, protected by the Delta works), bike infrastructure is as good as it gets (they are very big on cycling); i don't think you can find any place in Europe easier to cycle than the Netherlands, apart from maybe a little headwind at the coast
in most cities (Amsterdam, Rotterdam, ... very modern, a lot of museums) they have separate lanes (not just a white line somewhere, real lanes) for cars, pedestrians and bikes in BOTH directions, thats 6 lanes, its crazy, a bike is considered equal to a car somewhat, just about every bike road crossing has a pole with directions and distances to nearby villages so you hardly even need a map too

on the other hand its not as green and wild as Ireland, its a lot more populated (but traffic is never a problem due to the infrastructure),
and the weather is a bit more stable (nothing compared to dry/hot Greece though)
if you like culture, seeing cities, beaches, harbors, easy roads, cheese, then the Netherlands won't let you down
if you prefer a little more adventure, longer stretches of green, steep cliffs, hills, guiness, and are not afraid of rain; in that case Ireland is the better choice