Al-Qaeda put a $100,000 bounty on Vilks' head after he drew the cartoon, which was published in Nerikes Allehanda, a Swedish newspaper, on August 18, 2007.
Those arrested ranged in age from mid-20s to late-40s and were being detained at Waterford, Tramore, Dungarvan and Thomastown police stations.
Two police officers close to the investigation told the AP news agency that they were foreign-born Irish residents, mostly from Yemen and Morocco.
Under Irish law they could be interrogated for up to a week before being charged or released.
US authorities separately charged an American woman known as "Jihad Jane" with conspiring to kill Vilks.
The justice department announced the indictment on Tuesday against Colleen LaRose, who was arrested in October 2009, just hours after the Irish arrests.
Responding to Tuesday's developments, Vilks, who now lives under police protection, said he was unfazed.
"I'm not shaking with fear, exactly," he told TT, a Swedish news agency.
"I have prepared in different ways and I have an axe here in case someone should manage to get in through the window."
Vilks said he had received threatening phone calls from Somalia at the beginning of the year and that Saepo, the Swedish security police, had since advised him there was a heightened threat level against him.
"But I didn't think it was that serious," he told TT.
Mattias Lindholm, a Saepo spokesman, said: "Right now we are in continuous touch with the authorities involved, including our Irish counterparts."
Lindholm refused to comment on the threats against Vilks or on Saepo's actions to protect him.