In Dublin Centre on foot there are loads of great places to photograph:

Both people and urban scenes and all within 15 minutes walk maximum in any direction, people generally do not mind photographs being taken and are not agressive, for street performers Grafton Street and nightlife try Temple Bar and Wicklow Street areas.

Along the river for scenic shots and also good shots can be obtained from the rooftops of some inner city car parks. Every weekend there is some kind of protests or carnival.

It is literally a case of wander about till something takes your fancy. The bridges are all underlight in the Irish colours of Green, White & Gold
http://www.flickr.com/photos/13111789@N00/300560783/

Follow the river down the north quay walls until the harbour opens up and there are great morning shots their at sunrise.

The best public parks for shots are St Stephens Green, Merrion Square and Phoenix Park. All in walking distance.

Finally remember it is IRELAND so don't be afraid to ask security guards or others can you get access to their property or stand on their heads etc. to get better shots, they are not so uptight here and far more friendly and more than likely will let you