Are you coming from Galway? Around the M50 and then up the Drummartin Link Road is probably the best. You'll be going against the direction of the heaviest traffic. I do my best to avoid driving at that time of day but it shouldn't take much more than an hour from the N4 to Belfield.
To get onto the Drummartin Road from the M50 you need to make a funny kind of maneuver, you turn off at the exit marked Dundrum/Sandyford and then drive along a little road parallel to the motorway for a mile or two before turning towards Belfield at a big roundabout.
It's hard to describe in words but if you look at a map you'll see what I mean.
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