Telling the difference between Isis and Northern Irish paramilitaries should be fairly easy, though people do get into trouble for trying.
Some get reviled on the internet for demeaning the IRA with the comparison, others fall for trying to absolve the IRA as not being as bad.
One distinction that has been aired is that the IRA sought to avoid civilian casualties while Isis exults in them.
That's true up to a point. The IRA did often give warnings before bombs and experimented with anti-handling devices that would allow civilians time to get away.
The weakness in any claim to serious concern for civilians is that the many occasions in which their plans went awry and they killed themselves or others did not deter them from further bombing. They might have coined the term collateral damage themselves, for they accepted throughout their campaign that civilians were likely to die and carried on anyway.
And this takes no account of the many times they deliberately killed civilians, people who worked for the security forces in civilian roles, a census collector and those they branded as informers.
On top of that, most societies would regard police officers as civilians and the IRA killed more than 300 of them.
With the loyalists it is even plainer.
They, like Isis, set out specifically to kill civilians, seeking to terrorise the Catholic community into disowning the IRA, as if that were possible.
The big difference between our paramilitaries and Isis is that Isis is actually an army at war. Setting aside morality and concentrating on strategy, Isis is fighting a land war for territory, is holding territory, and governing it in its own brutal way.
In that regard it is not strictly a terrorist movement; it is a real guerrilla force with at least a notional prospect of winning, of taking territory and ultimately defeating its enemy's efforts to expel it from that territory.
The other contrast, of course, is that Isis bombers want to die.
That can't be said of many of the IRA or loyalist bombers, but in fact, in the early years of the Troubles, as many IRA volunteers died by their own bombs or guns as by any other means. Precisely half of IRA casualties were own goals.
And while people weren't queueing up to be martyrs, the rebel songs in the bars honoured the dead as martyrs for the cause, revering them in the same language which Isis uses.
Where Isis functions within the category of military activity that can best be called terrorism is when it strikes abroad, in France or Belgium or Britain.
There it is not seeking to hold territory or achieve military aims. It has no prospect of weakening the resolve or material power of the western powers which are attacking it in Syria and Iraq. If anything, it actually strengthens the resolve of those powers.
Its objective, and the usual objective of terrorism is propaganda - the propaganda of the deed.
Isis was eager to claim the bombing in Manchester because it would be no use to them if people didn't attribute it to them.
In ordinary war you don't need the credit for hitting the enemy so long as he has been hit and felt it.
That's the big difference between terrorism and crime; the criminal doesn't want you to know who did the deed, the terrorist does, and wants the act to advertise the cause.
In this, Isis in Europe and the IRA followed the same template. They were not fighting a war in any meaningful sense, to weaken an enemy and claim territory. Their violence is protest, not resistance.
Their objective is headlines and space on news bulletins.
The moral argument for some is founded on the cause. If you think that British rule in Northern Ireland was of such evil proportions that it warranted the IRA protesting against it by bombing pubs, furniture stores, shopping centres, then you can say it was a good thing.
You may hold that view and yet not think Isis has a similar right to protest against western powers in Syria by killing civilians in the west. You may consistently say that they are not the same, presumably because you think Britain's behaviour in Ireland was worse than the drone bombing of Raqqa.
Like Michelle O'Neill, you may commemorate some bombings and commiserate with others.
But if you assess the two organisations by their military methods and objectives, they have followed the same handbook.
Neither had or has the remotest prospect of crushing the British state by bombing pubs and clubs.
And neither, if it was honest, would say it was even trying to.