Best route from Dublin to Kilkenny via Glendalough

Can't help with a specific route, but we can share a cautionary tale on a similar plan (Dublin/Glendalough/Kilkenny) in May 2018.

We had the AA Atlas for Ireland, and I thought I'd studied it sufficiently. We were driving on a Bank Holiday weekend. We regretted not having GPS and made several frustrating errors south of Dublin before finding Sally Gap. Navigation was easy after that, but the roads were so jammed in/near Glendalough that we scrapped the plan to visit to ensure a timely arrival at our B&B. I suspect the heavy traffic was an anomaly due to the perfect weather and the Bank Holiday crowds at Glendalough.

We didn't find the driving difficult or the roads too narrow. With GPS and better planning than ours, you'll be fine. The conventional Forum wisdom is that driving times in Ireland will exceed predictions by G-Map and other tools, so I'd be prepared for 2 hours as a precaution. As I recall, the M9 was a breeze.

If you haven't already, you might want to investigate the toll issues near Dublin. Some rental cars cover it electronically. Ours didn't but we had coins ready. The Search engine above will lead you to lots of advice by searching "Dublin Tolls" or similar.

We stumbled on, and enjoyed, a nice restaurant (Horan's) in Baltinglass on N81, en route to the M9. https://www.horanspub.ie/home Any mid-sized town along the way is sure to have suitable dining options.

Did you take the Military Rd from Sally Gap all the way south to Glendalough?

Thanks for the advice on the tolls - I will look into that.

Military Road? Not sure, but I think not. Looking now at the map, I have a vague recollection of passing through Roundwood on 759. Military Road looks like more fun.

Toll advice: I mentioned the following in an earlier Forum post and am more than a bit embarrassed to repeat it, but here goes: leaving Dublin that morning we pulled into a wide toll lane and instinctively drove to the left side, looking in vain for the attendant or coin bucket. Of course, the attendant was far to our right, yelling "Over here! Over here!!" There was no way to maneuver sideways at that point, so he said "Just throw me something and get out of here." We've driven on the left side of the roads in England and Scotland, but I guess the coffee hadn't kicked in yet.

The Old Military Road (the R115) is quite pretty, especially the portion from the Sally Gap down into Glendalough, but it is one of those rural roads that Ireland is famous for, meaning that it will be narrow in places requiring one car of opposing traffic to give way in order to let the other pass. No big deal but you need to be mentally prepared for it when it happens to minimize the startle reflex of "what the heck do I do now" , especially in a vehicle as large as the one you'll be driving, ie one large enough to accommodate 5 people plus luggage.
It will be slow going but it is worth it.
Suggest getting the earliest start possible from Dublin, both to be able to drive at a leisurely pace and also to arrive at Glendalough well before the tour buses start to roll in around 9:30, which happens to be the same time that the visitor center (and bathrooms) open. The monastic site itself is always open and being able to stroll around it before the crowds arrive is a truly magical experience. There are some very pretty walks there too if you'd like to stretch your legs for a bit.
If you decide to drive via Glencree, consider a short stop there at the old milirary POW cemetary - a very evocative (and very pretty) place tucked back in the woods, As I recall there's a short walk along the stream to the site of an old Mass Rock nested back in one of the Glens.