See recent threads about US Dollars below ... Use your credit card as much as possible (not quite as easy in SA as in the US, but still pretty widespread acceptance of Visa/Mastercard) and get cash out of the ATMs there as you need it - No problem with finding ATMs in South Africa that take International Credit Cards
Getting Rand in Ireland before you go isn't a very cost effective option - You'll get a bad exchange rate and normally have to pay commission - so you'll lose at least 5% and possibly as much as 10%