Susan Finucane, who lived in Haiti for two years, says Concern has 120 staff in Haiti at the moment, six of whom are ex-pats.
"Most have been accounted for and they are ok, but we are worried about some who have not made contact, due to the lack of communication," she said from Concern's New York headquarters.
Finucane said most staff would have been out of the office, which closes at 3.30pm local time, when the earthquake hit at 5pm.
"Phone lines are down, internet access is sporadic, and the Haitians don't know much themselves, apart from what's going on in their nearby area. So we are only getting snippets of information," she said.
Concern is appealing for $7.5M in funding, and Finucane says that it is virtually impossible to gauge the extent of the damage.
"Our projects are obviously at a complete standstill. They are mostly in rural areas, and the housing structures are weak, like concrete shacks on hillsides.
"So if you see the damage done to the Presidential Palace, it doesn't bode well for the areas we work in."
Finucane will leave for Haiti with another member of the Emergency Response Team in the next two days.
The focus, she says, will be on providing water, shelter, food emergency kits, and plastic sheets to the local people.
Her first priority however, is to ensure all Concern staff are safe and well.
The Department of Foreign Affairs says it has now made contact with every Irish person they were aware of being in Haiti.
A total of 17 Irish citizens, made up of Digicel workers, aid workers, missionaries and tourists, are said to have contacted their families.