GPS coordinates if I want to travel through Sally Gap on way to Wexford from Dublin

Getting to the R115 (the Old Military Road) from the M50 motorway is a bit complicated since there's no direct exit for it - would require you to exit early and then work your way thru some semi-rural roads to find your way to the R115 - probably not something that you particularly need to be doing.
You might be better advised to continue around until you intercept the M11 heading south which quickly becomes the N11. That way you'd be on good quality divided highways for the first part of your driving journey while you're getting used to the whole dynamic of driving in Ireland. From the N11 I'd then suggest taking the R117 turnoff towards Powerscourt. It'll be well signed - you really can't miss it. You could then elect to stop at Powerscourt and tour the house (the gardens are nice but not in December), or continue past it and proceed west along the very rural L1011 towards the village of Glencree ... which might be a good place to stop for a stretch of the legs and a coffee. There's an old German POW WWII cemetary there - a very evocative place to wander around if you have the time and the interest. There's an old Mass Rock not far away, up a scenic little stream. The L1011 is one of those narrow, twisty turny things that Ireland is famous for, with barely enough room at times for two cars to pass side by side so you'll want to have your wits about you and mentally rehearse the pullover procedure in order to let approaching traffic to pass safely. Not that big a deal, but it does take some getting used to.
From Glencree you can pick up the R115 south thru the Sally Gap and on to Glendalough.
Per your other post about the best route from there on down to Wexford, I'd say your best bet is to proceed via the shortest route possible to get back to the motorway. After a long day negotiating rural roads the adrenaline will have started to wear off and fatigue will start to affect you. Best to plan ahead and to minimize your time on the road when that happens.
All of this is predicated on nice weather - the forecast for tomorrow looks fine for Dublin and Wexford but I wouldn't hestitate to just skip the drive to Glendalough if you decide (for any reason) that you're just not up to it.

This is great! OK I am printing this now! Thats true about getting the fastest route afterwards because I will also be tired from the overnight flight. Thanks!