Retailers have warned Irish holidaymakers after figures revealed that almost half-a-billion cigarettes were sold on the black market in Ireland last year.
One in every 10 packs of "roll your own" tobacco was also sold illegally in 2016.
An advertisement campaign by the Retailers Against Smuggling (RAS) group is warning those who travel abroad that they are breaking the law if they sell on tobacco purchased in another country.
The advertising campaign comes amid anger from legitimate retailers who claim they are losing a share of the market, despite paying taxes and excise duty on their products.
Posters will now be placed on buses on the Dublin to Cork Aircoach route.
RAS spokesman, Brian Carroll said: "Our campaign is designed to deter people who may be tempted to resell tobacco in Ireland without recognising the damage that this causes to shops nationwide."
Fianna Fail TD Declan Breathnach said the retail trade was being significantly undermined by the smuggling. Mr Breathnach added that there was also a resultant loss to the State on taxes.
"It's opportune that this particular campaign is focusing on people going to and coming from the airport and ports," Mr Breathnach said.
"Illegal importation of illicit products undermines the retail trade and diminishes footfall in our local shops, villages and towns.
"Those travelling abroad for this purpose need to be cognisant that they are assisting in reducing local revenue and employment opportunities, not to mention loss of revenue to our State to provide essential services."
A 2015 study by financiers Grant Thornton found that €2.4bn was lost in the economy due to illicit trading, 20pc of it due to illegal cigarettes.