The reality of the time was that, if huge numbers of catholics were fleeing to the Republic, the British army would have been working with the Irish army to both keep refugees safe and allow them to leave.
Even PIRA began to realise that the bombing campaign achieved nothing but pain,death, destruction, poverty and a hardening of views.
The UK government had left NI to govern itself without meaningful interference and it was the civil disobedience campaigns that highlighted the bias of NI society and lead to changes to reduce this. Sadly the bombing and murder campaigns made this harder to do as people were polarised. Once PIRA had learned that lesson, at huge costs in life and limb all round, then they moved to be involved in the government of NI and deal with the biases that existed and socially still do.
I do not excuse the loyalist groups who had devolved from formal organised self defence militias to being hijacked by gangs of bandits who eventually even sickened their own hard line members.
Equally I do not excuse the actions of soldiers going beyond the law but there are good reasons why armies are poor choices to cope with civil unrest. Young men, armed and trained as soldiers to be aggressive, in fear will not always act with wise restraint. This is why a civil police force is the preferred choice. USA experience in Iraq has shown the same sad consequences and I am sure that even the Irish army would find the same problem.
I urge the reading of Sir Charles Napier's papers in respect of his actions at the time of Chartist riots in England. It is a sound model for anyone to follow when acting in support of the civil powers. They are available on the internet and as downloadable ebooks.
More strictly on topic. The Irish Defence Force would mutiny and refuse to invade. If some did then they are more likely to have been arrested and returned home than blasted off the face of the earth. They would be more at risk of loyalist panic than of the British army who would surround them and share tea and fags until the politicians rejoined sanity.