You may be thinking of the rule that, if an employer engages an employee within 6-months of redundancy, then any application by the Company for State payment of the applicable portion of statutory redundancy will be invalid. So re-engagement within 6 months of the same employee would mean that it wasn't a real redundancy situation (in the State's eyes only) and the state reimbursement should be repaid (by the company).
If an employee is re-engaged within a certain period (I think it's 4 weeks), then their length of service remains intact (slightly OT but relevant nonetheless I think).
But, assuming there's a real redundancy situation at the time of redundancy, then the company can hire whoever they want, whenever they want after that.