CGT or CAT?
Either way, you complete and file your tax return(s). You instruct your solicitor to pay over the liability sum(s) to Revenue. You or your solicitor can then apply for tax clearance to enable the solicitor to pay the net proceeds to you. Without this clearance, they cannot do so and your money will remain in the solicitor's account until this is resolved.
If this is too much for you to handle, you can hire an accountant to look after all this for you. Or the solicitor may be happy to do this for you as part of their service.