21Proceedings for challenging validity of decision on appeal under s. 18
(1)If any person aggrieved by a decision of the Minister under section eighteen of this Act or the local planning authority desires to question the validity of that decision on the ground that it is not within the powers of this Act or that any of the requirements of this Act or of a development order or of the Tribunals and Inquiries Act, 1958, or rules made thereunder have not been complied with in relation to it, that person or authority may, within six weeks from the date of the decision, make an application to the High Court, and the High Court—
(a)may by interim order suspend the operation of the decision until the determination of the proceedings;
(b)if satisfied that the decision is not within the powers of this Act or that the interests of the applicant have been substantially prejudiced by a failure to comply with the said requirements, may quash the decision.
(2)Subject to subsection (1) of this section, the validity of a decision on an appeal under section eighteen of this Act shall not be questioned in any legal proceedings whatsoever.
(3)Nothing in this section shall affect the exercise of any jurisdiction of any court in respect of any refusal or failure on the part of the Minister to give a decision on an appeal under section eighteen of this Act.