Shaykh Dr Muhammad Umar Al-Qadri says that two mosques are seeing Islamic extremists congregating there regularly.
The Imam of the Al-Mustafa mosque in Blanchardstown also claims that there are now up to 100 extremists in Ireland.
He says that his group, the Muslim Peace and Integration Council, is working to promote peace but are meeting resistance in some quarters.
Speaking to the Irish Independent, he said: “We need to stop the radicals from spreading their lies to the youth.”
But he said that in one mosque, the religious leaders never denounce atrocities carried out in the name of the terror group Isil.
The Muslim community in Ireland is now about 40,000 strong.
Shaykh Al-Qadri warned that radicalisation of Muslim youth through both face-to-face and online interaction is becoming more and more prominent in Ireland.
Young people need to be educated about extremism to prevent this from happening, he said.
The day before the Muslim Peace and Integration Council held the ‘Not in Our Name’ demonstration against Isil in Dubin, the renowned cleric met three teenage, male Muslims in the city centre who supported the terrorists.
“I was not in my religious clothing so the boys did not recognise me as a Shaykh. I approached them and asked if they would come to the demonstration the next day. They said: ‘No, we hate Westerners, we are Isis’ (Isil). I was shocked.
“One of them then proceeded to quote a passage from the teachings of an Islamic scholar, which he thought justified violence against non-Muslims. I then asked him to give me another quote from that book and he could not. I kept asking him about this book and also about the teachings of the Qur’an.
“It turned out he had no clue about the teachings of Islam but was just quoting this passage from someone else.”
Shaykh Al-Qadri added that, at the moment, extremists in Ireland are non-violent and extremist in their ideology only.
He also said that there are no Sunni and Shiite tensions in Ireland and this would never be a source for violence here.
Asked how many individuals the gardaí were investigating for suspected terror links, a Garda spokesperson said: “For security reasons, we do not discuss operational matters of that nature.”