Treasa Ní Cheannabháin said she and her daughter, an Egyptian national, entered Gaza on Saturday, more than a week after Hamas militants knocked down the border wall.
Both women are members of the Galway branch of the Ireland Palestine Solidarity Campaign.
As hundreds of thousands of Palestinians flooded into Egypt, Ms Ní Cheannabháin, 56, said she sneaked into Gaza with her 19-year-old daughter, Naisrin el-Safty, to distribute money to needy Gazans.
Egyptian guards resealed the border on Sunday, ending the 12-day breach. Ms Ní Cheannabháin said she had not heard warnings that the crossing would close.
The pair tried to return to Egypt late Tuesday, but were stopped by Egyptian border guards.
The Department of Foreign Affairs is trying to help but only Egypt can authorise the pair’s return, a ministry spokesman said.
Meanwhile, at least 29 people have been killed and 16 injured when nine cars and lorries slammed into each other on a fog-choked rural highway in Upper Egypt.
Reports quoting police sources said a bus collided with a minibus and at least six other vehicles then rammed into them in the fog.