Gardaí seized a range of drugs within minutes of Electric Picnic opening on Thursday afternoon.
In a statement, the Garda Laois-Offaly Division said they had confiscated cocaine, ketamine, LSD and MDMA (ecstasy).
In a post of their official Facebook site, they said: "Gates opened at 4pm today [Thursday] and within minutes a number of seizures of illegal drugs have been made.
"Gardaí are carrying out searches and are reminding people attending that any seizures made will have consequences for the individual."
The post added: "Keeping people safe and free from harm is important. Cocaine, Ketamine, LSD, MDMA have been seized so far."
Gates to the Stradbally festival opened for early access ticket holder on Thursday, with events kicking off today.
Senior gardaí have separately told the Irish Examiner that anyone caught with drugs could face prosecution.
Detective Superintendent Brian Woods, attached to the Garda Drugs and Organised Crime Bureau, said gardaí will be conducting both "overt and covert" policing at the three-day festival.
Dr Eamon Keenan, head of HSE addiction services, warned users about the strength of ecstasy circulating and the risks posed by mixing multiple substances.
He said while drug use was not safe that the HSE was providing harm reduction messages to people who were going to use.
The Ana Liffey Drug Project is again providing welfare and outreach teams at Electric Picnic with harm reduction advice.