Hopefully not too late....
I really hate flying RyanAir, very no frills. But you can get cheap flights from London Stansted ( I don't believe they from from Heathrow) if you book it right it can be as cheap as £0.99 (not including taxes)
When driving out side of Dublin do not expect the roads to be of a quality of the US (although they are slowly getting better). Even though on a map Dublin to Cork does not look that far, you should expect to spend upto 4hrs driving.
In Dublin I would recommend taking a tour bus, they start on St Stephens Green at the top of Grafton St. You can see most of dublin city center in about half a day.
O'Connell St has also recently been revamped into a slightly more pedestrianized area so it looks alot less dull than it used to, still not much down that was to see though
