there are a lot of hidden gems in dublin city there is a podcast called 'hidden dublin' which has some interesting stuff. landscape wise in dublin itself howth head has something to offer,the deer park walk and the red rock walk may both be of interest to you there are secluded beachs,rocks and arboretums.
if you rent a car and drive an hour from dublin there county wicklow, places of interest are lugalla,glendalough,lough dan,avondale, brittas bay ,sally's gap, the vale of clara, camaderry and the wicklow gap etc all of this is very beautiful but you'll need deet because the boglands and mountains are full of midges.