A third of bus routes in the country are failing to meet their targets for arriving on time, according to the National Transport Authority (NTA).
The NTA said congestion is leading to only around 60% of buses showing up as scheduled.
Buses are deemed as being punctual if they leave not more than five minutes and 59 seconds later than planned.
In Dublin, 35 of the 112 routes missed their punctuality targets for 2018, with just 19 routes meeting all their targets.
Tim Gaston, Director of Public Transport with the NTA, said it is a problem across the country.
Mr Gaston said: "Particularly in cities, although not exclusively, we are also getting reports of congestion in some of the other towns around the country as well.
"But particularly in the city congestion has got noticeably worse in the last couple of years which inevitably impacts on bus services.
"Congestion is the main reason and the average journey times are increasing the average speeds are dropping."
In the capital, the 76A from Blanchardstown to Tallaght was the worst performing route.
Associate Professor Brian Caulfield, of Trinity College Dublin's Department of Civil Engineering, said the city's bus network needs to change.
Mr Caulfield said: "More priority, more buses, it's the one mode of transport we could double in the city in the morning, we just need to order more buses.
"It's not like a metro project where it takes a number of years to get that on board. BusConnects includes a huge amount of priority and that will be one of the biggest solutions and it's a turning point for this city.
"If we don't do it then we could end up with a city that we don't want."