There's a term in the computing world for software that's announced – usually more than once – but never gets released: vapourware. First proposed in 2005 as part of the government's Transport 21 plan for the capital, Dublin's Metro North, a mix of under- and overground railway, was to link the city centre with the airport. It's been announced more than once, but has to date remained the transport equivalent of vapourware. Last week, however, Minister for Transport, Shane Ross declared that construction of the €2.4bn project would start in 2021. Will it happen or is it déjà-vu all over again? And is it the best solution to Dublin's transport issues? Seán O'Rourke was joined by Associate Professor of Economics at Trinity College, Seán Barrett and Darragh O'Brien, Fianna Fáil TD for Dublin Fingal, to discuss the latest launch of Metro North. Seán Barrett starts his explanation of why he thinks it's a bad idea with a look at the project's cost-benefit analysis:

"Total cost is blank. Estimated net present value, blank. But they tell us the benefit-cost ratio is positive. We haven't seen the numbers."

Is €2.4bn not the number that came out last week? Seán asked. Well, where are the benefits then? Barrett replied. "We have to see €2.4bn worth of benefits to make it worthwhile." He warned that documentation on the project was scarce and cautioned that we have a long history of massive cost over-runs on construction projects:

 "The Port Tunnel went from €220m to over €700m."

Barrett's message to Minister Ross is: we need to have proper, open appraisal of this and all major infrastructure projects. "Put the thing up and let's see what the numbers are."

Fianna Fáil's Darragh O'Brien is very much in favour of Metro North. He argued that the fastest-growing area in the country is Fingal. He threw out two pretty impressive statistics: Dublin Airport is the fifth-busiest airport in Europe and will break the 30m passenger barrier for the first time this year, and Swords (in O'Brien's constituency) is larger than Limerick and has been identified in Fingal's growth plan as a major growth area.

 "People are so tired of hearing about announcement and re-announcement. This thing should be built now."

O'Brien, while very supportive of the idea of Metro North, believes the government is taking the wrong approach in delaying the start of construction for four years. He told Seán that Metro North already has planning permission from the original submission and the building could get underway tomorrow. Not only that, but he also said that the previous government sent back €500m of an approved loan from the European Investment Bank towards Metro North.

Seán (O'Rourke) put it to Seán (Barrett) that he has a history of opposing public infrastructure projects. Barrett was not bothered. "This will come," he said, "at the expense of waiting lists, at the expense of housing."

"The construction industry must be laughing all the way to the bank on this."

Darragh O'Brien (respectfully) disagreed with pretty much everything Barrett said. You can find out if they agreed to differ by listening to the entire piece on Metro North, as well as the rest of the Today programme here.