VAT3 and RTD from the same ledger
Most VAT-registered companies file a return for each two-month period, due by the 19th of the following month, with ROS filers given until the 23rd to file. Once a year the company also files a Return of Trading Details, which breaks total sales and purchases down by Irish VAT rate.
The less re-keying between those returns the better. Software that codes every transaction with its rate produces both from one set of figures.