Christian Aid says some places of worship will not be strong enough to withstand the Category 5 storm, meaning some families must be evacuated to cities.
Prospéry Raymond at the charity said: "Hurricane Irma is huge and we are very worried about the people in Northern Haiti and the Dominican Republic; particularly vulnerable communities."
The worst Atlantic Ocean hurricane ever recorded, Irma left at least eight people dead and a trail of destruction on Wednesday when it struck the islands of Barbuda, St Martin and St Barts.
The British Virgin Islands has also been buffeted, while there are concerns the British overseas territory of the Turks and Caicos Islands could be seriously affected last this week. A British naval ship with 40 Royal Marines has been deployed to the region.
The eye of the storm on Thursday morning was a short distance north of Puerto Rico where lashing rains and strong winds have already left 900,000 people without power. States of emergency have also been declared in Cuba and the US state of Florida.
World Vision, another Christian charity, said it was "poised" to help 122,000 people in Haiti with food, shelter, health and sanitation supplies.
John Hasse, World Vision's Latin America regional leader, said: "Our teams are poised to respond to everything that Irma has to throw at us.
"The threat of destruction is very real. Last year, I was in Haiti when Hurricane Matthew hit. Bridges were swept away and lives were lost."
Linking the disaster to climate change, Christian Aid fears a rise in temperatures in the Atlantic Ocean have made Hurricane Irma more powerful.
The British Overseas Territory of Anguilla was among the first Caribbean islands hit by Hurricane Irma. The UK said its "taking swift action to respond".
Foreign Secretary Boris Johnson said in a statement: "I've just spoken to the Chief Minister of Anguilla to discuss the devastating impact of Hurricane Irma. My thoughts are with all those affected and the UK is taking swift action to respond.
"We have staff and a British naval ship ready to help those in need. Brits should follow our travel advice which will be regularly updated."