If it was me I'd probably leave the R109, turn right onto the R121 just before Lucan, and head up on the north side of Leixlip, there is a small unclassified road on the south side of the Westmanstown Golf club, joins up with the R149, then left on the L1014, and head west on the R148, head for Enfielf and then south west to join your route via Endenderry, this is just because the traffic can be pretty heavy out through Lucan and Cellbridge, the road north of Leixlip misses out most of this, you'll be heading passed the Intel Factory, (west side of Leixlip marked as Collinstown Industrial park) so not so good if you are planning to be there at shift change (7:20 and 19:20). I lived for a year in the early '90s on the south of Pheonix park, and usually rode my bike to work at Intel in Leixlip, it was quite a good ride, I always liked the bit by the Liffey under the M50 bridge, used to be a good boozer along there,
http://theanglersrest.ie . I was working in the area earlier this year, traffic is still busy on the west side of Dublin, but since the motorway through to Galway has been opened most of the back roads are fairly quiet, and many of the small towns on the way always seem to have pretty good chip shops, and a pub or two that will do decent cycling food.
