from the UK....
Flights are very cheap £20-£100 with Ryan Air, British Midland and British Airways
Ferry/Rail £84 for two people on a Dublin Duo with Virgin - very good value for money, but takes from London around 8 hours in comparison to 1 hr on the plane
Site seeing bus, hop on hop off £7.50
Taxi - fairly reasonable
Flights to Dublin are available from £0.49 before tax.
With tax added it means you can fly with Liverpool's finest for around £50 return for two.
Still one of the cheapest ways to go to Dublin is Ryan Air. You can fly to Dublin from London and about a Dozen other Cities all over Europe.