A celebration is due to take place at a restaurant in Co Donegal over the weekend, after charges linking the 62-year-old to the 1982 attack were thrown out by a judge last Friday.It later emerged that the case against Mr Downey had collapsed because he had mistakenly been sent a letter telling him he wasn't to be prosecuted as part of a secret government scheme.Up to 500 people, including senior Sinn Féin politicians Gerry Kelly and Pat Doherty, are expected to attend a party on Saturday to welcome him home after nine months in prison.But the gathering has been criticised by unionists who have branded it "ghoulish".Danny Kinahan MLA, of the UUP, said: "Sinn Féin really is rubbing salt into the emotional wounds of the family and friends of those who died on that terrible day in 1982.This is almost literally dancing on the graves of those who were blown apart, whilst on ceremonial duties.Danny Kinahan"This man should be on trial in a court of law, not living it up in a pub in Donegal. It is in incredibly bad taste and brings shame on those who are organising the event."TUV leader Jim Allister said: "This ghoulish celebration will sicken people across Northern Ireland and the rest of the UK. It is a telling reminder of the nature of Sinn Féin and their total insensitivity to victims and their unfitness for government."Meanwhile supporters of Mr Downey have defended the event saying it is for a man who never should have been arrested.Seamus O'Domhnaill, a Fianna Fail councillor who knows him well, said: "We would welcome the decision that he be released, given the fact that the proceeding document (judgement) produced ratified that he should never have been arrested."Four British soldiers died when an IRA car bomb detonated in South Carriage Drive as they rode through the park to the changing of the guard.The revelation that Mr Downey was sent a secret letter sparked a political row in Northern Ireland and Prime Minister David Cameron has now announced a judge-led inquiry into the circumstances surrounding 187 letters sent to so-called 'on-the-runs'.