One of the city's most rewarding walks is a stroll along the Great South Wall to the Poolbeg Lighthouse, the stumpy red tower visible in the middle of Dublin Bay. The lighthouse dates from 1768, but it was redesigned and rebuilt in 1820. To get here, take the 47 bus from the city center to Ringsend and then make your way past the power station to the start of the wall (it's about 1km).
It's not an especially long walk out to the lighthouse – about 800m or so – but it will give you a stunning view of the bay and the city behind you. It's best enjoyed just before sunset on a summer's evening.