Whole route for all Three Sisters is about 4 hours. Ascent is quite easy for experienced walkers but bear in mind some parts of the ground may be like a sponge so have a right gear on the right place. There is no hiding place in case of rain except the last "sister" where you can hide inside the Seefin Passage Tomb.
Climbing Seahan gives you spectacular view to Dublin, Kippure, Wicklow Mountains and Seefin and Seefingan caches. Take a while on the hill to find around the marker some other tombs built here in Neolithic times. http://www.megalithomania.com/show/site/42/seahan_hill.htm
Inside the cache I placed some items related to the SEA I brought from the beaches on the West Coast of Ireland.