Hi tinkerbell
When you arrive in JHB, there are connecting flights to Nelspruit with airlink aprox 1 hour of flight time. This would make your journey to either Kruger Park, timbavati, Sabi Sabi and so many more game parks to visit that much easier and will save you at least a 6-7 hour car journey plus toll raods to the game parks. Most of the private game parks are around Kruger Park. You will more than likely have to rent a car and this could be done in Nelspruit with either budget or avis car rental. you will need a international drivers licence which cost around €5 from the AA(valid for one year) but remember to take your irish drivers licence as well.
I think 3 days is plenty for a safari trip.
The journey from Nelspruit to anyone of the game parks is around 2-4 hours if they are in the Mpumalanga province depending on how far north you want to travel. Nelspruit airport also has flights direct to Capetown where you can enjoy the garden route, wine farms and table mountain.
The Mpumalanga province ie Lowveld area has loads to offer ie Safari, shopping at the riverside mall, Quad biking, water rafting, mountain biking, more shopping, walking/hiking with amazing waterfalls and landscape.....do a google search for lowveld, gods window, bourkes luck potholls, three rondawels., pilgrims rest, graskop, hazyview. There is also plenty of accomodation in these areas offering what ever budget you may have.
On another note , the distances between towns can be quite high, however the road structure is very good and although you might be stuck in a car for 2 hours, the distance travelled could be far. to give you an idea, the distance from jhb to nelspruit is around 370km but this can be done in 3.5 hours if the traffic is favourable....
Anyway i could go on and on, so feel free to PM me if you have questions, i would be happy to help..
cheers
Brianp