The Revenue will not be able to track these deductions if the principals have not submitted the RCTDC details. They cannot therefore give him credit for the tax deducted. The principals are obliged under RCT to deduct the tax, remit it to Revenue and give the sub-contractor the RCTDC. If they have not done so he needs to chase them up. If he has names, addresses of the principals he should write them, requesting the relevant RCTDC's to be forwarded to him immediately ( as in within 7 days) failing which he will have to submit their names/addresses to Revenue for them to follow up and enable him to get his deduction. Revenue are usually pretty strict on RCT, I'd get a contact name for an officer in his district and try deal through them.