If you only get one stop alone in Dublin, make it Howth Cliff Path Loop (free, 7.8 km loop from the DART, ~2 h). Next on our list are Phoenix Park (free 24/7, 700+ ha, wild fallow deer) and National Museum of Ireland (free, bog bodies + prehistoric gold); the other three, down to St Stephen's Green, are below with where to book.
free, 9-ha Victorian park, from 7:30am
free, 3.4 km walk to a red lighthouse
free, Vermeer + Caravaggio, late Thu
free, bog bodies + prehistoric gold
free 24/7, 700+ ha, wild fallow deer
free, 7.8 km loop from the DART, ~2 h