Sir Bernard Lovell last night became the first man to examine the skies above Ireland through the restored Birr telescope at Birr Castle, Co Offaly.
Yesterday a new six-foot mirror and accompanying optical system were installed at the refurbished telescope in the castle grounds.
From its construction in 1845 until 1918, the telescope was the largest in the world. But in 1918 the mirror, which had been made by craftsmen at the castle, was removed. Since then the telescope, known as "The Leviathan of Parsonstown", has been unused and had fallen into disrepair, until a restoration plan was implemented.
Last year the restored wooden structure was brought back to the site, and yesterday a new mirror, of nickel-coated aluminium alloy and weighing a mere 1.5 tonnes (half that of the original) was lifted into place.
The work was supervised by Mr David Brooks, an optics expert with University College London. He has been working on the mirror for almost a year.
The original mirror is on display at the Science Museum in London, and no moves are being made to have it returned to Ireland.
The provision of the mirror was funded by a grant of £200,000 from the Department of Arts, Heritage and the Gaeltacht.
Shannon Development grant-aided the Historic Science Centre at the demesne. The telescope will be the centrepiece of the science centre as it remains the largest reflector telescope in western Europe.