Fáilte Ireland has said at least 10,000 jobs would be lost in the tourism sector in the event of a no-deal Brexit.
The agency says a UK crashout from the EU could lead to the loss of more than one million British tourists each year, resulting in the loss of €380 million per annum.
Some 3.5 million visits from Britain were recorded in 2018, contributing around €1bn to the Irish economy.
The agency says it is undertaking more research on the possible impacts of a hard Brexit on the tourism market and expects to revise the possible losses "considerably" upwards in the coming weeks.
Fáilte Ireland's Head of the Wild Atlantic Way, Miriam Kennedy, said the challenges facing the sector were very stark.
She said extensive efforts were under way to assist the tourism industry, with advice on revenue management, diversification and new markets openings.
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Ms Kennedy said an over-dependence on the British market would leave businesses exposed and said a round-the-clock campaign was under way to ensure people were as prepared as possible for all eventualities at the end of October.
She was speaking as those involved in the sector in the west gathered for an industry training day, at which new data on emerging and existing markets was presented.
Businesses are being urged to ensure they are fully versed on the options open to them. There are particular fears about the impact a drop in off-season British tourists could have.
Fáilte Ireland has conducted extensive research on the ways in which tourists from Europe and the US can be attracted to Ireland, with a particular focus on the French and German markets.