Can't give any advice about Orlagh estate, but I've lived in Knocklyon for the past 5 years and I love it. Nothing negative to say about the place atall.
Traffic is average but hopefully will improve once they finish the M50 and the new bridge there as well. Buses have improved immensely with the 15s now running alot more frequent plus the 74 and 74A running every 15 mins between them.