Hundreds of B&Bs and guesthouses in the southwest have been approached by the Revenue Commissioners in "an ongoing project" asking them if they are fully tax compliant.
The warnings are part of an approach to "cash" industries in the southwest, including hairdressers and fishermen.
More than 300 owners of B&Bs and guesthouses have been written to in Cork, Clare and Kerry. They are being asked if they are in compliance in terms of declaring income and if they need to regularise any issues, including if they needed to register for VAT.
Most of those selected have been B&B owners, but a small number of hotels were written to between July and September. There are up to 1,000 B&Bs in the area. In Kerry alone, there are 100 registered guesthouses.
A Revenue spokesman has declined to say how the selections were made or whether only registered premises were targeted.
Hundreds of B&Bs in Kerry are not registered with Fáilte Ireland, and do not appear on official brochures. However, most advertise on the internet and on local tourist free-sheets.
The spokesman said those targeted were not suspected of having done "anything wrong", and the trawl was "a sectoral approach".
"It doesn't mean there is anything wrong. Instead of looking at individual businesses, we are taking an entire sector - as well as looking at individuals."
Other sectors which had been targeted recently in the southwest include hairdressers and the fishing industry. Some hairdressers in Kerry have paid substantial amounts to make themselves compliant.
Tourism industry chiefs are advising those who contact them to fully declare any cash held in accounts.