Anyone who refuses to wear one will face fines of up to €2,500 and time in prison.
The rule will not apply to children under 13 or anyone who can't wear one due to physical or mental incapacity.
Retail staff will be required to wear one unless there's a partition or space of two metres between them and customers.
High levels of compliance with mask-wearing have been reported across the country, since they became mandatory in shops.
Retailers say they've noticed compliance has steadily improved, and are urging the public to help make shopping safe for all.
