Think putting an ATR rather than an Airbus320 on the Brussels route is more realistic than optimistic I'd say.
Next summer Aer Lingus will operate Dublin to Brussels 17 times weekly (11 x A320 + 6 x A321), surely 3 x A320 from Cork is not that unimaginable.
There is evidence of demand for a Cork to Brussels route if you look at the August 11th poll on Cork Airport's Face-book page:
Cork Airport - Airport - Cork, Ireland | Facebook
However I do expect the gap in the schedule will be filled with Berlin or Madrid.