The woman was detained on Tuesday as part of a group of seven suspected of plotting to kill Vilks, who has a $100,000 bounty on his head from an Al-Qaeda-linked group.
Three men still remain in custody in Ireland, while two other women and a man were freed without charge on Friday.
Police said files on all four suspects freed so far will be prepared for the Director of Public Prosecutions (DPP), meaning charges could still be brought.
"A female arrested on March 9 and detained at Thomastown Garda (police) station (in southeast Ireland) has been released from custody this afternoon," said a brief statement.
The seven original suspects included three Algerians, a Libyan, a Palestinian, a
Croatian and a US national, a police source told the news agency AFP. They ranged in age from mid-20s to late-40s.
This week, the Irish Independent newspaper reported that an eighth suspect in the plot, "Jihad Jane", the online name of Colleen LaRose, spent two weeks in Ireland last September on a "fact-finding trip" before her arrest in October. According to a report from the Washington Post, she also travelled to Sweden in August to locate Vilks and plan his murder.
LaRose has been indicted in the US for recruiting jihadist fighters in the US, Europe
and Asia in a bid to carry out terror plots.
The US Justice Department has declined to confirm whether LaRose was connected to the alleged plot to kill Vilks.