There are many reasons that an aircraft type can be changed - it doesn't necessarily mean it's due to the Virgin announcement.
Besides, the Irish Times suggests they're sourcing new airframes for the Virgin deal:
It is understood Aer Lingus will source additional aircraft to service the routes, bringing its short-haul portfolio up to 40 aircraft.
The EI website currently lists 37 short-haul planes in the fleet (the two A319s are the newest, both having entered service in March of this year). So perhaps one plane from the current fleet will be used.
Unless there were routes that they wanted to cut anyway, it would seem to be bad business sense to pull planes from the existing network to cover this new commitment. Presumably Virgin bears all or most of the "risk", so EI can afford to lease new planes to fulfil the deal. Why didn't Virgin do that themselves? Because they have no crew certified for operating, or maintaining, shorthaul planes.