In the case of refunds to customers that's a reduction on sales so VAT (if the sale was VATable) would be due back to the seller on that amount.
If you miss a sale in a period and discover it in the next or a subsequent period, if the amount is not major in the context of your overall sales, then the VAT can be handled in the period in which it is discovered. However, in strictness, you could submit a supplementary VAT return for the period in which it should have been accounted and pay the amount due. Revenue are entitled to interest on late payments. There may be plusses and minuses over the year and once they are not material Revenue allow a common sense approach ot be adopted in handling them.