Rotterdam is certainly an airport facing many difficulties. Noise pollution regulations have put a brake on further expansion of the airport. The opening of the HSL rail line from Amsterdam to Brussels will mean that Rotterdam is only 20 minutes by train from Schiphol.
However, yours is the first suggestion I've heard that it's going to close in recent years. There was a government plan to close it in the 1970s to use the land for housing, but my understanding was that this was formally scrapped about 5 years ago.
However, I can't really see a Cork-Rotterdam link working. Aer Arann doesn't really have enough brand recognition in The Netherlands to get passengers from Rotterdam to Cork. While my experience is that Rotterdam has better nightlife, restaurants and shopping than Amsterdam, it doesn't have a knock out attraction to draw tourists in (while Amsterdam has several - the museums, the Red Light District etc.). So, Rotterdam would probably have to be reliant on business traffic and when Aer Lingus have an almost double-daily A320 service into Schiphol, which will soon be only 20 minutes from Rotterdam, business passengers will choose to use this rather than a 3 or 4 times weekly ATR service from Aer Arann.
On the subject of Aer Arann at Cork, I remember hearing last Autumn that Cork was likely to see a service to Bordeaux this summer. The route doesn't seem to have materialised, but at the time, I had considered that Aer Arann were the most likely operators. Even now, looking at their route profile, it's one that they could probably successfully add. It's far enough from Nantes that it shouldn't impact yield there and is in a part of France that has traditionally been popular with Irish tourists, but is unserved by any airline.
Also, with Aer Arann, I'm a bit surprised we haven't seen a frequent flier programme from them in their fight against Ryanair on the Dublin route. For regular travellers who are spending their company's money on the flight, it could provide a useful incentive.