Dundrum to Citywest would be an easier cycle (and more cycle paths) if you got onto the Grange Road and just keep going straight along there until you hit The Mill pub on the junction of Firhouse/Tallaght. There are a lot of cycle paths if you go that way and you avoid obvious busy places like Templeogue.
Then take the second left after the Mill and at the next roundabout you can keep going straight through Killinarden or turn right and then left at the end to go along the Tallaght bypass (either way you end up on the main road beside Jobstown - which is perfectly safe to cycle along).
Then keep straight until you come to the junction for Citywest.
Its a long cycle!
For locking your bike around Tallaght, you'd really have to nosey around to get a good spot but maybe in the grounds of the college, or the carpark of the Square?