The three, two women and one man, were freed after three-and-a-half days of questioning. Three men and one woman remain in custody.
The group was arrested Tuesday over an alleged plot to assassinate Swedish cartoonist Lars Vilks, who has a $100,000 bounty on his head from an Al-Qaeda-linked group.
"They were released without charge," an Irish police spokesman told news agency AFP.
Those arrested were three Algerians, a Libyan, a Palestinian, a Croatian and a US national, a police source told AFP on Thursday. They ranged in age from mid 20s to late 40s.
Suspects can be held for up to a week after their arrest without charge. Police are preparing files on the three who were released for the country's director of public prosecutions, meaning they could still face charges.
The controversy started when Swedish regional daily Nerikes Allehanda published Vilks' satirical cartoon in 2007 to illustrate an editorial on the importance of freedom of expression.
The cartoon prompted protests by Muslims in the town of Örebro, central Sweden, where the newspaper is based, while Egypt, Iran and Pakistan made formal complaints.
An Al-Qaeda front organisation then offered $100,000 to anyone who murdered Vilks - with an extra $50,000 if his throat was slit - and $50,000 dollars for the death of Nerikes Allehanda editor-in-chief Ulf Johansson.
On Wednesday, leading Swedish newspapers published Vilks' cartoon again in a demonstration of solidarity.
A second American Muslim woman has been arrested in conjunction with the case, The Wall Street Journal reported late Friday.
The newspaper identified the woman as Jamie Paulin-Ramirez, 31, who worked as a medical assistant in Leadville, Colorado, but recently moved to Denver and then to New York.
Earlier this week, the Irish Independent newspaper reported that a suspect known as "Jihad Jane", the online name of Colleen LaRose, had spent two weeks in Ireland last September on a "fact-finding trip" before her arrest in October.
LaRose has been indicted for recruiting jihadist fighters in the US, Europe and Asia in a bid to carry out terror plots.
She was reportedly monitored with a couple in Cork and Waterford in southern Ireland, where the seven were arrested.
US prosecutors said that LaRose had agreed to carry out the murder of a Swedish resident, pledging "only death will stop me."
The US Justice Department has declined to say if LaRose was connected to the alleged plot to kill Vilks.