The work, which will provide up to 150 jobs, is expected to be completed in time for Christmas Eve Mass in 2014.
Chairman of the cathedral project committee, Séamus Butler, said, “There was a huge sense of loss when it burned, people were crying.”
He now hopes a school of specialist construction skills can be developed during the restoration.
“That is the legacy, it fits also with the acquisition by the local authority of Connolly Barracks.”
The cathedral was gutted by a fire on Christmas Eve in 2009. The blaze destroyed the roof and much of the interior, and continued burning into Christmas Day.
On Monday Bishop of Ardagh and Clonmacnoise, Colm O’Reilly, signed contracts for the project with Longford-based GEM Construction and Galway-based Purcell Construction, which together will restore the cathedral.
“After all of the planning and design, it is particularly pleasing to have arrived at the stage of having contractors on site so that we will now begin to see the great effort of the past few years bearing fruit,” he said.
Bishop O’Reilly said work “has been ongoing behind the scenes by St Mel’s cathedral project committee, diocesan art architecture committee and the design team to ensure the cathedral is restored as a place of beauty, welcome and prayer.”
Specialist skills such as lime mortar plastering, ecclesiastical decoration, specialist cleaning, stone fixing, brass and metal work will all be required in the project.
Pre-apprentices will learn conservation and repair of masonry and be given an introduction to traditional roofing, slating and leadwork.
The course award is by the Scottish Qualification Authority.
Mayor of Longford town Cllr Peggy Nolan (FG) said the contract signing was “very positive . . . It’s a very, very good news day.”