DART+ (DART Expansion)

DART+ between Parkwest and Heuston is not comparable to between Killester and Raheny. Parkwest to Heuston has working space and full accessibility along the northern side as far as Inchicore. Then there is the CIE owned Inchicore works to the south. A track may have to be closed to allow for working space on the trackbed, while rail operations continue on the other two tracks. They'll probably put up hoarding to separate the works area from the railway. You also don't have OHLE to worry about which significantly reduces the risk of operating machinery based the tracks and makes closing and reopening a track easier and quicker.

West of Inchicore is more constrained but is generally wider and less steep embankments. That whole section needs to be reengineered anyway, including adjusting trackbed, realigning tracks, OHLE cables and supports, new signaling and drainage, etc. Even if not widening, you would still have major works there just more electrification so including the widening works doesn't add hugely to the complexity or disruption.

And I didn't say that additional tracks on the Northern Line was impossible but it would involve significant disruption to rail operations and major CPO of private houses. I suspect that the report will say lots of things are possible but the full impacts of them would have to be assessed in further detail. Basically kick the can down the road in the hope the consultant gets another gig out it.