A lot. There's a road at both ends of the runway which would need rerouting or tunnelling. Terminal while functional for a 70 seater is far too small for a 189 seater.
The extension that never happened was apparently still too short for a fully laden 738, it was priced at >€25M.
The M9 and N25 are both killing and capable of saving WAT - they makes the airport a lot closer to a lot of areas that would be consider its service area; but it also makes them a lot closer to DUB. The old N9 was a cart track, the worst national primary in the country.