If you prefer a shorter route, I recommend the triangle Tallaght – Blessington – Sally Gap – Tallaght (46 km). I did it as my last ride there. (see Strava)
My main point by writing this post is that you'd better pay attention to the (changing) weather conditions.
My intention was to go off-road after Sally Gap, in order to climb up to the Kippure peak. It is about 250 meters above the highlands plateau.
Otherwise the weather was its usual self – sporadic rain and sporadic sunshine – but quite soon after Blessington the wind rose.
On the treeless mountains wind was blowing hard. Whenever it was headwind, I almost came to a standstill even on downhills. When it was side wind, I struggled to keep upright and on my lane.
As my friend had warned me previous evening (thanks, Robin) that the very peak of Kippure is notorious for high winds, I saw the mission impossible and continued wrestling along the road.
Then came also the rain. Being wet and windy, it felt really cold! I'm more than happy that I was able to put on a raincoat, on top of my rain vest. Putting it on must've been a funny sight: the super light coat was very very difficult to get into, as the wind was dragging it to another direction all the time.
So, add to the list of necessary items (this post): a raincoat.