A cheque is potentially legally valid for an indefinate period, there is nothing in any of the Bills of Exchange Act that says it goes stale after 6 months. However the acts do refer to a cheque not having been in circulation for a considerable period of time( or words to that effect). Therefore, it is normal banking practice to unpay cheques that are more then 6 months old but there is no legislation that backs that period up.
In addition, most banks don't check the dates on small value cheques, especially if they were lodged at other branches as it is not cost-effective to do so. (Bear in mind that over 1m cheques are still issued in Ireland per week).
By all means raise a complaint with the bank, but you could be depending on their goodwill to get anywhere