A C2 is a certificate issued by the Revenue which means that the holder can be paid gross by contractors. Otherwise, Relevant Contracts Tax must be deducted at source. It has nothing to do with tax clearance certs or public sector contracts
Releatively speaking, having a current C2 it does mean that they are tax compliant, as the C2 would be withdrawn, should they fall into tax arrears. As a previous poster advised, says nothing about quality of work. I would be more drawn to recomendations from previous satisfied customers than current C2.