The ecstasy tablet was three times the normal dose.
The pill contained 312 milligrams of the drug, which the health service says is enough to kill someone.
A HSE drug alert was issued to the phones of all festival goers, through its app, when it identified the tablet.
Back-of-house drug checking at festivals - which allows people to anonymously deposit a drug sample for analysis - has been in operation for a number of years.
