The number of visitors coming to Ireland from the US and Canada fell 8.8% in October, compared to the same period last year.
It follows a drop of 4% in September.
The trend will be of concern to those involved in the tourism industry, who have been trying to grow the US market in the face of Brexit.
In contrast, the quantity of visitors from the UK increased by 4.2% during the month, despite uncertainty surrounding the Brexit process.
However, latest figures from the Central Statistics Office show that so far this year, the number of British visitors is only just positive, up less than 1%.
In total, 956,800 people came here from abroad during the month, an increase of 2.5% compared to 12 months earlier.
336,400 of those came from Britain.
This brings the total number of visitors to the country since January to 9.34m, up 2.2% compared to the same period in 2018.
Growth from other European countries was a lot stronger than that experienced from Britain during the month, rising 8.3% to 355,900.
Residents here made 737,000 trips overseas during the period, up 2.3% on a year earlier.
Tourism Ireland said the decision by Norwegian to discontinue its transatlantic services in September meant the removal of almost 5,300 airline seats per week between North America and Ireland.
"The figures very much reflect feedback from our industry partners on the ground, who've seen weaker demand this year," said Niall Gibbons, CEO of Tourism Ireland.
"The continued uncertainty around Brexit is giving rise to consumer concern in Britain and some Mainland European markets. There’s also been a deterioration in air access capacity this year."
"The additional funding allocated to Tourism Ireland in Budget 2020, in response to the Brexit challenge for Irish tourism, means that Tourism Ireland is rolling out a number of end-of-year kick-start promotional campaigns right now – in the US, Britain, Germany, France, Spain, Italy, the Netherlands and the Nordic countries – to kick-start our promotional effort and position us well for 2020."
The Irish Tourism Industry Confederation CEO, Eoghan O'Mara Walsh, said the fourth straight decline in visitors from the US and Canada is of particular concern.
"North America has been the main driver of growth for Irish tourism in recent years and success there needs to be consolidated," he said.
"It is very disappointing that there is a decrease in overseas marketing budgets for Tourism Ireland in 2020 with a softening market, Brexit and competitiveness challenges at play - this is particularly so given the Vat hike on the tourism industry this year".