Organisers of the The Battle of Clontarf Festival say that more than 40,000 people have turned out for the first day of the weekend event at St Anne's Park in Raheny in Dublin.
500 living history re-enactors from all over the world are taking part, with two Battle of Clontarf re-enactments a day.
Participants in the re-enactment began to arrive and set up camp in St Anne's Park on Tuesday.
The Battle of Clontarf re-enactments are the largest such re-enactments to have ever taken place in Ireland.
A medieval village has also been set up in the park with over 80 tents where many of the combatants are living for the duration of the event.
There are also skills and weapons demonstrations, and a Viking longboat is on display.
Organisers had estimated that 40,000 people would attend the event hosted by Dublin City Council over the course of the weekend, however the fine weather has meant that this figure will be significantly higher.
Entry to the festival is free. It runs from 11am to 7pm on Saturday and Sunday, with re-enactments at 1pm and 4pm each day.
People are advised to use public transport, with additional busses running on both days, however there will be no DART service on Sunday.