This is getting into crayonism.
There are plans for a line from Tallaght via Kisoghue (Kildare Line) Porterstown (Maynooth line) which provides the link. It not going to be a heavily used route
Given current outlook, looks unlikely to go ahead in the short term, that said the land purchase cost has collapsed in recent weeks making it a lot cheaper.
Any further discussion on Metro West, this thread
http://www.railusers.ie/forum/showthread.php?t=2692 please