The crypt of Daniel O'Connell at Glasnevin Cemetery in Dublin has been refurbished and will be open to visiting groups every day from now on.
President McAleese, who officially opened the monument this afternoon, said O'Connell was not just a major figure in Irish history, but also in British and world politics.
Known as 'The Liberator', Daniel O'Connell was one of the great historical figures of Ireland and lived from 1775 to 1847.
He secured emancipation - greater rights and freedom - for Catholics in 1829.
He died in Genoa while on a pilgrimage to Rome. He requested that his heart be taken to Rome and the rest of his body to Ireland.
O'Connell's coffin has been entombed since 1869 by a large altar stone of black Kilkenny marble with the Durrow Cross inscribed into it.
The coffin can be seen and touched through a number of portals cut into the stone. A tower above the crypt is one of Dublin's landmarks, standing 170 feet tall.
Ten members of O'Connell’s family are also laid to rest in the crypt.
A suspected loyalist bomb attack resulted in the monument being closed in the early 1970s. Two decades later an attempted restoration was carried out, but it failed to combat the problem of dampness.
Now a full renovation has been completed by the Office of Public Works.
President McAleese said this afternoon that O'Connell's relentless advocacy helped ignite the push for human rights and democracy that would eventually transform the politics and political structures of the twentieth century.
The President said that during her honeymoon, she and her husband Martin visited the O'Connell monument in Glasnevin.