Quick answer: Dublin runs about EUR 160 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-June and September - mildest weather and long evenings, with fewer crowds than July-August.
What things actually cost
cappuccino at a cafe
4 EUR
pint of Guinness
6.50 EUR
Book of Kells entry
18.50 EUR
bus or Luas ride with Leap Card
2 EUR
pub lunch main
14 EUR
mid-range dinner main
20-25 EUR
Eat well for little
Pint of Guinness at a proper pub like Mulligan's or Kehoe's - about 6.50 EUR
Full Irish breakfast - about 14 EUR
Fish and chips from Leo Burdock - about 10 EUR
Seafood chowder with soda bread - about 10-12 EUR
Toastie and a pint at Grogan's - about 12 EUR combined
Money mistakes to avoid
Drinking only in Temple Bar - pints run 8-10 EUR there versus 6-7 EUR in normal pubs a few streets away
Not pre-booking Kilmainham Gaol - tours sell out days ahead and walk-ins are rarely possible
Getting a taxi from the airport by default - the Dublin Express and Aircoach buses cost about 7-8 EUR and take 30-40 min
Overpacking the schedule - the center is small, leave time for a slow pub evening, which is the point of Dublin
Transport without the tourist tax
From Dublin Airport the Dublin Express or Aircoach buses reach the center in 30-40 min for about 7-8 EUR. The center is very walkable - for longer trips get a Leap Card for buses, the Luas tram and DART, with most rides around 2 EUR.