The volume of retail sales fell by 1.9% in September compared with the previous month, according to the Central Statistics Office.
The fall followed a strong rise in August. The volume of sales was up 3.2% compared with the same month a year earlier.
The value of sales - which takes prices into account - was 1.6% lower in the month, giving an annual increase of 4.7%.
A big drop in motor sales was a major factor in September's falls. The volume of bar sales fell by more than 4% in the month, and they are now 8.5% lower on an annual basis. Sales of electrical goods fell by 5.7% in the month.
Davy Stockbrokers said the monthly figures were very volatile, and pointed out that retail sales growth picked up in the third quarter as a whole, rising 1.1% compared with Q2.
It said garage sales were particularly strong in the quarter, but the September bar sales figures were 'awful'. Davy said the volume of bar sales had now fallen by more than 9% since the smoking ban came into effect.
