The Olympic torch has toured several locations around Dublin, culminating in a public celebration in St Stephen's Green.
Thousands of people also turned out for events in Howth, Croke Park and O'Connell Street.
The torch is visiting Ireland as part of the 70-day London 2012 torch relay and crossed the border from Northern Ireland early this morning.
Dublin is the only city outside Greece and the United Kingdom to host the flame.
The torch was passed between two former Olympic boxers, Belfast's Wayne McCullough and Dubliner Michael Carruth, during a special ceremony at the border.
Thousands then lined Howth Harbour in Co Dublin to greet the Olympic torch on its first official stop in the Republic.
Local student Cillian Kirwan from St Fintan's High School in Sutton had the honour of running with the flame into Howth, where he was greeted by President Michael D Higgins and Lord Sebastian Coe of the London Olympics.
President Higgins told the large crowd the flame's arrival in Ireland symbolised the growing closeness of the relationship between the UK and Ireland.
After a brief ceremony where the Olympic hymn was sung by schoolchildren from Howth, the torch was brought under garda escort to Croke Park - where Kilkenny hurling star Henry Shefflin, representing the GAA - carried it along Croke Park's skywalk.
A collection of sports stars and other famous faces carried the torch throughout the morning.
Soccer star Paul McGrath received a warm reception from the crowd, as did singers Jedward who ran with the torch along O'Connell Street.
The flame was finally brought to a public celebration at St Stephen's Green where Sonia O'Sullivan lit the Olympic Cauldron to commemorate the visit.
Organisers say the trip south was aimed at illustrating both the success of the peace process and the all-island nature of the Irish Olympic team.