CSO figures show number of tourists from Britain declined by 3 per cent in first quarter of year.
The numbers of key, high-spending US tourists visiting the Republic grew by more than 5 per cent over the first three months of this year, compared to the same period last year.
The US tourists, who traditionally stay longer and spend more than their Continental or British counterparts, are particularly important for regional tourist enterprises outside Dublin.
However, the number of tourists from Britain declined by 3 per cent, reflecting an overall decline in the number of Britons travelling abroad.
The figures published by the Central Statistics Office (CSO) also show the undefined "other markets" were down by 16 per cent.
The figures have surprised observers, who had expected the war in Iraq to hamper severely US trade.
In other markets, continental Europe fared well, with visitor numbers up by 11 per cent.
Overall, while the number of international tourists who spent a night in the Republic in the first quarter of 2003 grew by just a half of one percent, earnings rose by 9.7 per cent to €722 million.
Tourism Ireland stressed the figures were for tourists - those who spend at least one night in the Republic - as opposed to visitors, who may arrive and leave on the same day.
When the figures for both overnighters and day-trippers were combined, a slight decline of about 0.3 per cent in overall numbers was recorded. This possibly reflects less same-day business trips as a result of the economic downturn.
The actual number of combined visitors and tourists who visited Ireland in the first three months of this year was 1.11 million.
Meanwhile, the number of Irish travelling overseas grew by 5.4 per cent to 939,000, according to the CSO. Irish tourists abroad continued to spend more than inbound tourists - €749 million against €722 million, the CSO found.
Predicting tourist numbers is becoming more and more difficult, however, largely due to late booking on the Internet. According to Mr Paul O'Toole, chief executive of Tourism Ireland, "the Internet is playing an ever increasing role in consumer decision-making and last-minute offers are the order of the day".
The figures were cautiously welcomed by the Minister for Arts, Sport and Tourism, Mr O'Donoghue, who said they represent a good performance in a volatile market.
Extra funds for marketing campaigns have been made available this year and the campaign is aimed at capitalising on the upturn in consumer confidence in the wake of the Iraqi conflict.
The marketing programmes for Britain, continental Europe and the US have all been geared up since the end of the war "to ensure that Ireland remains very much to the fore in the minds of those people who may be finalising their holiday plans for the summer and autumn", Mr O'Donoghue said.