President Michael D Higgins has laid a wreath at the spot where 15 rebels were executed for their part in the 1916 Easter Rising.
In a poignant ceremony at the Stone Breakers' Yard in Dublin's Kilmainham Gaol, the President was flanked by a military guard of honour drawn from the Defence Forces cadet school.
Prayers were said before acting Taoiseach Enda Kenny invited the head of State to lay the wreath "on behalf of the people of Ireland in honour of all those who died".
After a minute's silence, a lone piper from the Army No 1 Band played the lament Wrap The Green Flag Around Me Boys, before the Last Post was sounded.
The Irish Tricolour, which flew at half mast throughout, was then raised.
The simple ceremony was among the first in a national day of events commemorating the tumultuous insurrection which ultimately led to the partition of Ireland and the creation of the Republic.
In what will be the largest public spectacle in the history of the state, about a quarter of a million people are expected to line the streets of Dublin for a military parade lasting several hours in honour of the revolutionaries.
The most historic site will be the reading of the Proclamation under the portico of the GPO on O'Connell Street around midday - a re-enactment of the actions of rebel leader Patrick Pearse on Easter Monday 1916.
After the Proclamation is read the President will lay a second wreath at the GPO with scores of descendants of the rebels looking on.
Government ministers have been keen to stress that the centenary celebrations are carefully designed to honour the courage and ideals of the Rising and its leaders and reflect all of Ireland's history.
President Higgins used an address to descendants on Saturday to call on Irish people to take responsibility for building a true Republic and said the ideals of the Proclamation can still inspire today.
Hundreds of thousands of people will line a 4.5km route across Dublin from 10am for the parade.
Some 3,722 Defence Forces personnel will march in front of military vehicles along with emergency services personnel and army veterans, many of whom served on United Nations' peacekeeping missions.
The Air Corps will also perform a fly past.
The commemorations began on Saturday morning when Sabina Higgins, the President's wife, lays a wreath at the grave of Countess Constance Markievicz in Glasnevin Cemetery.
Following that, the President laid a wreath in the Garden of Remembrance in honour of all those who fought and died for Ireland's freedom.
Along the route of the parade 22 viewing screens have been erected and five setback areas are planned for families to congregate.
Organisers have warned of massive demand for public transport and access to the city, but with trams not running due to a strike by Luas workers the capital will be facing unprecedented demand for bus, Dart and taxi use.
The parade is expected to end at around 3pm.
On Easter Monday further commemorations are planned at each of the seven key battlefield sites in Dublin.
Wreath laying ceremonies, again open to the public, will take place at the 1916 garrisons including Boland's Mill, the Jacob's Factory on Bishop Street, Dublin Castle and City Hall, the Four Courts, the Royal College of Surgeons, Moore Street and at St James' Hospital, which was the South Dublin Union 100 years ago.
Outside of Dublin simultaneous wreath laying ceremonies will be held in Athenry, Cork, Enniscorthy and Ashbourne.
Also on Easter Monday cultural events are planned in more than 200 venues across Dublin city centre including 500 free talks, exhibitions, debates, film, performances and dramatisations, with six outdoor stages and lots of activities for children and families.