The seasonally adjusted figures for retail sales fell by 3.3% last month when compared to the month of July, according to the Central Statistics Office.
The figures show an annual increase of 2.6%.
Without Motor Trades, the data shows there was an increase of 0.8% in retail sales in August when compared with the previous month and there was growth of 4% in the annual figure.
The sectors with the largest monthly volume rises were Other Retail Sales (+7.3%) and Food, Beverages & Tobacco (+2.9%).
The sectors with the largest month-on-month falls were Books, Newspapers & Stationery (-4.0%) and Motor Trades (-3.9%).
There was a decrease of 1.9% in the value of retail sales in August 2018 when compared with July 2018 and there was an annual increase of 1.9% when compared with August 2017.
If Motor Trades are excluded there was an increase of 0.6% in the month and an increase of 3.2% in the annual figure.