The Tanaiste, Ms Harney, represented Ireland at a Requiem Mass in Madrid last night for the two Spanish victims killed in the bombing.
Ms Harney, who interrupted her holiday in Portugal for the second time this week, flew to the Spanish capital shortly before the Mass in the Nuestra Senora del Recuerdo College chapel, the school attended by one of the victims.
She was received at the airport by the Spanish Foreign Minister, Mr Abel Matutes, who had arrived from his holiday home on the island of Ibiza.
The Infanta Elena, eldest daughter of King Juan Carlos, and her husband, the Duke of Lugo, attended the Mass on behalf of the Spanish royal family and visited six of the injured children in hospital.
The British government was represented by Mr John McFall, the Secretary for Health and Social Affairs at the Northern Ireland Office.
Other Spanish dignitaries included the Education Minister, Ms Esperanza Aguirre, and Mr Ruiz Gallardon, president of the Autonomous Community of Madrid, a close friend of the Blasco family, whose son, Fernando (12), was killed in the blast.
Families of the Spanish victims have all praised the care they have received at the hospitals and from the public in Northern Ireland. "They have done everything for us," said Mr Jos Manuel Blanco, whose son, Gonzalo (12), was until yesterday in a Derry hospital, when he was transferred to join his sister, Teresa (13), in Belfast.
"An Irish volunteer has never left our side; it is as if I had a personal chauffeur driving me from Belfast to Derry and back whenever I wanted to travel."
Gonzalo Blanco told Spanish reporters that he remembered little of the explosion, "but at least I am alive". Among his many gifts he is particularly happy with a Liverpool football shirt and a fax from his hero, Michael Owen. "When I get back I am going to frame it."
In a statement, the families of Rocio Abad Ramos and Fernando Blasco Baselga, the two Spanish who died, said: "We are very grateful to the Spanish and British governments and to all the organisations and everyone in Spain, Britain and Ireland that we have come into contact with, for their welcome, support and love they have given us at all times.
"We wish to highlight the great efficiency shown by the management of the Donegal Centre, in particular Paula Helguero (Mrs Mahon), who have spared no effort to assist us.
"We would also like to express our gratitude to the Irish families with whom our children have stayed for their affection over the years. We would like the deaths of Fernando and Rocio to be the last ones caused by terrorism and their deaths to shed a ray of hope on the paths of peace." Three of the Blasco family's six children were studying at the Donegal Centre this summer. A daughter, Lucrecia (13), is recovering from serious facial injuries, while Juan Pablo (10) was uninjured and has decided to stay and finish his course.
Mr Blasco knows about terrorist bombs from experience as he was injured when an ETA bomb exploded near his car as he was driving in Madrid six years ago.