The library collection from the 800-year-old Howth Castle is being sold at auction over the next two days with books dating from the 1500's, along with rare manuscripts and letters.

The contents of the castle were auctioned earlier this month when an 18th century portrait of the writer Jonathon Swift was sold for €250,000.

The most valuable item in the library sale is a complete original copy of Ippolito Rosselini's history of Egypt, including three volumes of illustrated plates.

George Mealy of Fonsie Mealy Auctioneers said: '’This is probably the most important literary sale to happen in my lifetime.

"Not only are there exceptionally rare and valuable items, there are also items of immense importance to Irish and British history that would be of interest to dozens of academic institutions."

Among the 1,100 lots are the private correspondence of Thomas Gaisford, Dean of Christ Church, Oxford, which includes an archive of letters to Reverend John Henry Newman, later cardinal, now saint, and correspondence with five former British prime ministers.

George Mealy said this gives "an insight into the movers and shakers that he was involved with and the influence at government and college level at the time".

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The Howth Castle Library sale includes the complete collection of the Botanical Magazine from 1794 to 1983, which contains 10,750 hand-coloured illustrations.

Fonsie Mealy Auctioneers said this was the only record of such a complete collection anywhere in the world today.

Each generation of the Gaisford St Lawrence family continued to the subscription for almost 200 years.

There is also a 1767 original manuscript document granting the Viscount and Earl of Howth to Thomas Gaisford St Lawrence.

"This is one of the true original documents connecting the family to the castle", said Mr Mealy.

Howth Castle has its origins in medieval times.

Howth Castle has been renovated extensively over the years (Pic: RollingNews.ie)

In 1177, Almeric, the first Lord of Howth, came to Ireland with John de Courcy.

Legend has it that on the feast day of St Lawrence he won a victory that secured him possession of the Howth peninsula and in gratitude for this he took the name of St Lawrence.

The castle was extensively altered by succeeding generations to adapt it to their times, most notably in 1738, when the house took on its current appearance and again in 1911 when Edwin Lutyens renovated and added to the house.

In 1909, the last Lord Howth died, and his nephew Julian Gaisford inherited the castle.

More than 800 years after setting up home in the Dublin coastal village of Howth, the Gaisford-St Lawrence family has sold the historic castle and its 470-acre estate to Irish investment group Tetrarch in a multimillion-euro deal.

The online auction began at 10.30am and will continue tomorrow.