What the regulations say
Under the Food Safety and Standards (Labelling and Display) Regulations, 2020, 'best before' is the date signifying the end of the period during which the food stays fully marketable and keeps the qualities claimed for it. After that date, the food may still be safe, though quality may have dropped, but it must not be sold once it actually becomes unsafe.
'Use by' or 'expiry' is the date after which the product may no longer be safe, and the food 'shall not be sold or distributed for human consumption' after that date.
What this means for you
Treat 'use by'/'expiry' dates as hard limits: do not buy or consume food past this date.
'Best before' is about quality, not a guarantee of safety either way. Use your senses (smell, appearance, taste) as well.
Date of manufacture or packaging, and the expiry/use-by date, must be grouped together on the label under the 2020 Regulations.