They were not murdered by the Irish State or even anyone from it. Werent the murderers from Northern Ireland? The UK claims NI, so officially speaking, they were murdered by their fellow British subjects as part of a domestic dispute.
How do you know who killed them? There were plenty of members of the IRA from the Republic.
They were killed by members of the IRA who were proclaiming themselves as the true army of the Irish State. They received plenty of support from the South including arms and finance. Plenty of attacks were launched from this side of the border. We even elect members of the IRA to represent us in the Dail down here. I won't even go into Haughey and the Arms trial.
You can try and call it a 'domestic dispute' but the death of innocent Children hundreds of miles away from any war or dispute is as much a stain on our History as Bloody Sunday is on the the UK's. We may not have approved of their actions but the IRA acted in the name of Ireland. Just like the British Army acted in the name of Britain despite the fact that most civilian people would have been horrified at some of their actions.