1. At the outset, you need to realise that once this man died, all potential legal claims for compensation died with him. So whilst his estate can potentially sue for the five weeks injuries, because the man is dead and cannot speak for himself, little or no damages will result, as courts do not believe in giving a windfall to dead people or their estates. So you should judge your next action in the light of this basic legal fact. His estate might potentially have a claim for wrongful death against the employer but given he died of a heart attack, there would have to be clear evidence that his death was actually caused by the accident before recovery could be made. Be aware that the damages for wrongful death by relatives have a statutory cap in Ireland.
2. You can report the wrongful actions of the work Health & Safety officer to the HSA. They can issue a direction that the injury be properly noted in the work records, on the correct date. Additionally, they can penalise the employer for this default. Disciplinary proceedings can be initiated by the employer against the Health & Safety officer on this basis. Be aware that there is no obligation on the Health & Safety officer to report the accident or the outcome to the Gardai or police.
3. So you should make a complaint to the HSA and formally start the grievance procedure at work on behalf of your dead colleague.