While the Green Luas line is nominally built to Metro standard, it has a heap of road junctions. It seems rather pointless to have a Metro line that's completely grade separated from Swords to Stephen's green, but has to deal with road crossings south of that. Plus you would leave the extension to Broombridge orphaned.
My fantasy "Metro South" would continue underground at Stephens Green, heading south-west, under Harold's Cross, head south under Terenure, break ground west of Terenure and run as an elevated railway roughly following the path to the Dodder through Tempelogue before joining the N81 out to Tallaght, with potential for extension to Jobstown and Citywest.
Yes, that would potentially result in three rail routes terminating in Tallaght, but Tallaght and Swords both have large centres of population as well as significant industrial areas, so connecting both to the city centre, along with a lot of population centres not currently served by rail, makes a lot of sense.
Yes, I know it's a total fantasy route that will never happen, but we're all entitled to our fantasies.