I agree with our gang try and do as much online researching prior to coming
then you can concentrate on finding the records not available online!
The
Public Library at Pearse Street is a great source for electoral registers, newspapers, cemetery records on microfiche
The National Library as mentioned is a vast resource of information and their website has great tips for before you visit
http://www.nationalarchives.ie/ may also be of interest
Local libraries and historical societies may be of interest depending on where your family originated from.
I suggest that you ask yourself what do you
most want to achieve from the trip?
Is it more information on the family?
Is it to locate missing family members?
or their graves, birthplaces, etc.?
Is it to see where they lived?
or maybe all of the above!
The answer will guide you in the correct direction for planning
I would also suggest you research opening times and locations for the places you are going to visit, and before you arrive plan your days to make the best of your time.