As part of DART+ Southwest, they're building a new station, Heuston West, where the disused Heuston Platform 10 (the one with the through lines) is now. It'll. be on the DART line and provide service to, presumably, Grand Canal Dock, via the Phoenix Park Tunnel (some commuter trains from Grand Canal Dock already use the tunnel and pass Heuston, but platform 10 hasn't been in use since before the tunnel was reopened). I'd assume that Houston and Heuston West will be linked. Won't be done til 2029, tho, and if any part of it slips it will be building the new station (Irish Rail is historically bad at that).