01 Blarney is not a quick detour from Dublin
A day tour solves the transport problem, but it cannot remove the distance. Twelve hours is the minimum realistic shape of the outing, and adding Cahir or Cobh pushes more of the day into scheduled movement. That can still be good value when you have one day and no car; it is not the same as seeing Cork slowly.
At Blarney, the queue for the Stone can consume a meaningful part of the stop in busy months. If kissing the Stone is non-negotiable, go there first and explore the gardens afterwards. If it is not, the grounds and Rock Close are often the better part of the visit.
02 Cork, Cahir and Cobh tell different stories
Cork gives you a living city and a food-market stop. Cahir is a compact, defensible castle beside the River Suir. Cobh is a harbour town shaped by emigration and transatlantic travel. None is filler, but each pulls the day in a different direction.
This is why a comparison page is more useful than a list of supplier titles. The right tour is not the one with the highest number of stops. It is the one whose second and third stops match the reason you are travelling south.