Key documents and historical treasures relating to the turbulent birth of modern Ireland are due to go under the hammer later this year in what has been billed as "the Irish sale of the century".

Scheduled for mid-April to coincide with the 90th anniversary of the 1916 Easter Rising, when Irish rebels staged an ill-fated insurrection against British rule, the auction will comprise nearly 500 lots, many of them previously unseen.

The star attraction, according to Dublin auction house James Adam & Sons, will be the original words and music to Ireland's national anthem. It is expected to fetch up to 1.2 million euros (824,000 pounds).

"That would be the highest price ever paid for an Irish historical document," said Stuart Cole, a director of Adam's, which is co-hosting the sale.

"But these things are almost impossible to value because they are so emotive -- and you don't get much more emotive in that sense than the national anthem."

Handwritten by Peadar Kearney in 1907 on two pieces of paper, the "Soldier's Song" was popularised by Irish rebels during the 1916 Rising and formally adopted as Ireland's national anthem in 1926.

The uprising, in which nearly 500 people were killed and thousands injured, was a military disaster for the rebels -- whose leaders were subsequently executed -- but proved to be an overwhelming symbolic victory, paving the way for Ireland to become a fledgling state six years later.

Cole said awareness of the value of 1916-related items was raised by the sale last year of a surrender note by rebel leader Padraig Pearse which sold for 700,000 euros -- more than 10 times the estimate.

The note's purchase by a private collector, believed to be from Europe, angered many who want the Irish government to play a bigger role in keeping such artefacts in the country.

Cole said he hoped a lot of the material to be sold on April 12 -- much of it put up for sale by families directly involved in the battle for Irish independence -- would stay in Ireland.

Other lots expected to attract interest include poignant letters written by rebel leaders before their execution and a telegram informing Irish secretary of state W.T. Cosgrave that King George V had agreed to give Ireland independence.

An Irish flag believed to have flown over Dublin's general post office, where the rebels holed up against British artillery fire during the uprising, and a typewriter belonging to revolutionary Michael Collins are also up for grabs.

"No sale of such national importance has ever been held before, and we imagine it won't be matched for a long time after," Cole said.