In June 2015, transport engineers working on behalf of the government prepared the Fingal / North Dublin Transport Study (NTA & AECOM, 207 pages). It examined 25 potential options for providing a high-capacity transport system for Swords and North Dublin. They examined options like extending the Luas Green Line from Broombridge to Swords, creating a new bus rapid transit system and extending the DART line out to Swords. The report concluded that a metro system to Swords and North Dublin was the best option for building a high-capacity public transport system for North Dublin.

Swords is expected to grow from a population of 46,000 (Census 2016) to over 100,000 in the coming decades. Ballymum is also expected to grow significantly. Dublin Airport is expected to grow from 27.5M passengers a year (2016) to 36M passengers by 2033. The Dublin Airport business park is also expected to grow significantly. It is clear that we need to dramatically increase the public transport options in North Dublin.

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Currently the overwhelming number of trips in the area are taken by private car. This will be unsustainable into the future. Traffic will grind to a halt and extra pollution from added cars will be damaging to our health and the environment. Transport modelling predicts that by 2033 journeys from North Dublin into the city center will take 72% longer than they do now if no public transport infrastructure improvements are made.

The Fingal / North Dublin Transport Study examined 25 potential options. From the initial 25 only 6 options were brought forward for detailed analysis.

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The 6 options were then analysed on their technical feasibility, their impact on the environment, their safety benefits, how cost effective an investment each project would be, would the option encourage social inclusion by providing service to those less well-off and on how well the option integrated with other transport and local and national planning.

The report concluded:

... the key transport infrastructure priority for Fingal/North Dublin is the provision of new public transport capacity for the corridor from Dublin City Centre, through the north inner city and Ballymun to the Airport and Swords. Investments in this corridor provide the greatest community and economic benefit. - Page 203

The study found that the heavy rail option from the Maynooth line would be very constrained. Without reducing service on the Maynooth line it would be impossible to run more than 4 trains an hour to the airport and North Dublin. This provides insufficient capacity. The heavy rail spur from Clongriffin to the Airport and Swords did not provide a transport option for the people of Ballymum. It was found to be a poor public investment. It would also add capacity limitations to the Dublin/Belfast rail line, potentially limiting the expansion of the DART to Skerries and Balbriggan.

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None of the bus rapid transport options could not provide the necessary capacity to serve North Dublin.

The light rail option of tunnelling from the Green Line Luas Cross City section near Broombridge to the Airport and north Dublin is fairly similar to the Metro North option in the study. However, the on-street running of the Green Line adds capacity constraints, and a tunnel from Broadstone to the city center would also need to be built to expand capacity shortly after opening. This made this option very similar to the Metro North option but with many disadvantages. Extending the Luas Green Line towards Ballymum and Swords also removes the option to expand the Luas Green Line up to Finglas.

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The comprehensive report identified that option LR7 Metro North was the best medium and long term public transport solution for the Greater Dublin Area. This report was the reason that Metro North (the old name of the MetroLink project) was included in the NTA's Greater Dublin Area Transport Strategy 2016-2035.

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You may notice that the route map above is different to the MetroLink route map. During the MetroLink planning process a full route alignment study was carried out. It looked at 40+ potential route options for the metro from the city center to north of Swords.

You can read the full Fingal / North Dublin Transport Study here. This will give you more in-depth  reasons for why each option was eliminated and the Metro North option was selected as the best option.