Hi Geri, I had a similar situation with a house I rented out (although in my case I needed it back to live in), I was surprised too to discover I had to give six months' notice, it seemed excessive to me. It ended up a bit messy in the end (I posted about it earlier), dragging on for another few months because the tenants couldn't find anywhere to live within their budget (they were also paying lower than the market rate in my place).
If you can't hold on until April of next year then, as Gordon said, your only option would be to offer the tenant a financial incentive to leave, maybe a contribution towards the difference between what they are paying in rent now and what they would pay in a new place? There's no way around that notice, so have a word with them and see if you can work something out.
Just in case you're unaware of this, make sure this is included in the notice of termination (from citizensinformation.ie):
If the property is being sold: If the landlord intends to sell the property within 3 months of the termination of your tenancy, the notice of termination must state that “The reason for the termination of the tenancy is due to the fact that the landlord intends to sell the dwelling, for full consideration, within 3 months after the termination of the tenancy”. The notice must also include a statutory declaration stating the landlord’s intention to sell.
Best of luck, I hope everything works out - if you can wait until April at least you're likely to get more for your property, so there's that consolation.