Air traffic between both capitals also far exceeds the next busiest routes of London-Amsterdam (3.6m) and London-Paris (2.9m).
The latest official EU figures are likely to add further pressure on the Government to speed up approval for the construction of a second terminal at Dublin Airport.
They also reveal that Dublin was the eighth busiest among airports in the 15 EU member states in 2000 with almost 11.6m passengers - a growth of 7.2% on the previous year.
Flights between Ireland and Britain rank third within the EU, accounting for just over 4% of all intra-EU traffic after UK-Spain (12%) and Germany-Spain (9%).
Remarkably, over 43% of all passenger traffic within the EU involves flights to and from Britain, although this is believed to largely be explained by Britain’s island status.
According to official EU figures, the number of travellers passing in and out of Ireland grew by 17.6% between 1993 and 1999 - the second highest growth rate in the EU after Sweden.
London-Heathrow remains Europe’s busiest airport with more than 24.3m passengers during 2000 followed by 22.8m for the combined traffic of Charles de Gaulle, Orly and Le Bourget airports in Paris and 21.3m at Schiphol Airport in Amsterdam.
A total of 53.4m travelled through the four London airports - Heathrow, Gatwick, Stansted and City - in 2000.
The figures reveals that the Cork-Birmingham route is also one of the fastest growing within the EU, recording a 79% increase in 2000 - the fourth biggest percentage increase between any two destinations.
However, three routes from Ireland also featured in the top 10 experiencing the biggest percentage decline in passenger traffic in the same year.
Flights from Dublin to East Midlands and Madrid showed one of the biggest percentage declines, with numbers on both routes falling by 27%. The number of passengers between Kerry and London also dropped by 22%.
Traffic from Ireland to non-EU destinations accounts for just over 1% of all passengers, a reflection on the small number of destinations apart from the US which have direct flight connections with Ireland.
Overall, air traffic across the EU also grew in 2000 to a record 434m people, an increase of 8.7% on the previous year.
However, figures for 2001 are expected to show a general decline in air travel as a result of the September 11 terrorist attacks on the US.