There was a reason for all that.
(What's the difference? Longer carriages, angles of turns, and strength of foundations.) irishtimes.com/news/luas-line…
But why stop there when there's already a Metro-grade line running south from there? Why not link them?
It took the old Metro North plans and extended them, building an underground line from Swords to Charlemont and then taking over the existing Luas line southbound from there to Sandyford.
From Sandyford to Cherrywood remains a Luas; the Luas Cross City from Charlemont to Broombridge remains a separate line of its own.)
This is true of ANY upgrade-the-Luas-to-Metro proposal.
Where the plan gets complicated is where the existing Luas crosses over a road. A Metro track is simply too busy to drive over.
THIS is the closest the existing Luas comes to being "dug up".
This means less of the existing Luas gets the upgrade it needs, and more duplication between old Luas and new Metro.
It has not been stated exactly WHY this would mean a four-year disruption.
As already mentioned: aside from station upgrades, the main overground works are building the 'fork' to allow the Metro run onto the metro-grade existing Luas line.
The policy question that poses: is that worse than closing off one road just south of Ranelagh?
The southern part of Metrolink will probably end up being shelved, and only Metro North will go ahead for the time being.