President Michael D Higgins rang the Peace Bell at Áras an Uachtaráin earlier to remember the victims of the Omagh bombing.
Victims were also remembered at a special service in the Co Tyrone town on Wednesday.
This marked 20 years to the day since an IRA car bomb exploded in the town, killing 29 people - including a woman pregnant with twins.
A memorial service was also held on Sunday.
Dissident republicans, opposed to the Northern Ireland peace process, had planted the 500lb bomb.
Three children from Co Donegal and two Spanish tourists were also killed and nearly 300 others were injured.
With no one convicted by a criminal court, the bereaved and injured have been campaigning for justice for two decades.
People gathering on Market Street, Omagh, ahead of the ceremony to mark the 20th anniversary of the Omagh bombing on August 15th 1998 | Image: Niall Carson/PA Wire/PA Images
The President and Sabina Higgins rang the Peace Bell at 3.10pm, marking the exact time of the explosion 20 years ago.
The ceremony at Áras an Uachtaráin took place with the approval of the families of the victims, mirroring a similar event in Omagh.
President Michael D Higgins today rang the Peace Bell at #ÁrasAnUachtaráin, in honour of the victims of the Omagh bombing.
See https://t.co/8ssgWIKMTr pic.twitter.com/Zqf9SD4qmk
— President of Ireland (@PresidentIRL) August 15, 2018
In a statement, President Higgins said: "On the 20th anniversary of the Omagh bombing, we remember the 29 women, men and children who were killed, and the hundreds who were injured, in a horrific act of violence.
"As President of Ireland, I wish to add my voice to the expressions of solidarity with the people of Omagh and to take the opportunity to acknowledge again the courageous work of the first responders of 20 years ago.
"As we recall with deep sadness the deaths and the injuries at Omagh, we should use this moment to recognise the many ways in which we are all connected and the many ways in which we have sought to make peaceful progress over the last two decades.
"May we use this anniversary as we honour the victims, to re-emphasise the importance of building a culture of peace and tolerance on our island.
"While the implementation of the Good Friday Agreement remains a work which requires our continued vigilance and commitment, its principles - so overwhelming endorsed by the people - represent the best hopes for the future of our shared island.
"On this poignant anniversary, all of our thoughts must be with the families of those affected and with all those who continue to work towards a future of peace and reconciliation."
People gathered on Market Street, Omagh, during the ceremony to mark the 20th anniversary of the Omagh bombing on August 15th 1998 | Image: Niall Carson/PA Wire/PA Images
Tánaiste Simon Coveney has said his thoughts are with the people of Omagh.
"The spirit of the families and community of Omagh and their commitment to reconciliation - as was shown at Sunday's ceremony - are an inspiration to us all", he wrote on Twitter.
My thoughts today are with the people of Omagh, especially the victims and survivors of the atrocity of 20 years ago. The spirit of the families and community of Omagh and their commitment to reconciliation - as was shown at Sunday’s ceremony - are an inspiration to us all. pic.twitter.com/NgmmQffG4U
— Simon Coveney (@simoncoveney) August 15, 2018
A bell sounded 32 times in Omagh - one for each of the 31 victims of the bombing, and once more for all those who have lost their lives to terrorism around the world.
The bell sounds 32 times-once for each of the 31 victims of the Omagh Bombing, and once for those who’ve lost their lives to terrorism around the world-on the 20th anniversary of the atrocity in the County Tyrone town. pic.twitter.com/0uI4CbvI5x
— Q Radio News (@qnewsdesk) August 15, 2018
People also cast petals into the water at Omagh Memorial Garden.
People cast petals into the water at Omagh Memorial Garden, on the 20th anniversary of the Omagh bombing, in which 29 people, including a mother pregnant with twins, lost their lives. pic.twitter.com/2IlqB01FYT
— Q Radio News (@qnewsdesk) August 15, 2018
Additional reporting: Stephen McNeice
