Almost 3 million euro has already been spent on engineers and consultants for the Bus Connects plan.
Newly released figures published in the Irish Times show a US based consultancy firm, that won the tender in 2016, has been paid over €600,000 for work up to August this year.
Another 11 consultancies have shared €2.2 million to date for their work.
There has been widespread confusion about the Bus Connects proposal, which would transform the look, layout and frequency of routes across the city.
Sinn Fein's Transport Spokesperson, Imelda Munster, is concerned about the outsourcing of work.
She described the figure as "shocking", adding that "You have to ask where is the in-house?".
Deputy Munster said it "certainly puts into question the setting-up and funding of State bodies such as the NTA and TII if they are void of any expertise".