The emergence of a video of protestors trying to attack the convoy of President Michael D Higgins, as he left a school in Finglas last Friday, has raised some eyebrows - and quite a lot of commentary.

In particular, the description of the President as a "midget parasite" has concerned many - with the Taoiseach this morning joining the chorus of those who describe the actions as unacceptable.

Leo Varadkar has meanwhile called them an "attack on the constitution".

But those behind the protest clearly see it as justified - believing they are entitled to vent their grievances at President Higgins approving the various laws permitting water charges.

While I'll leave it up to individual readers to decide whether the protest is appropriate, the incident does perhaps warrant a reminder of the exact powers the President has.

As the Taoiseach said on a interview with Newstalk this morning, the Presidency "has always been above party politics" - largely because Ireland adopted a presidency as a direct replacement for the monarchy, which always been apolitical.

But that's not the whole point of the story. The President's prime function is to make sure the Constitution is upheld - and that Constitution, in turn, only gives him a very limited scope to act on any particular law.

In fact, the President has no discretion to personally veto any bill presented to him for his signature. The Constitution describes his role very specifically.

It says that - with the exception of a few other instances, which we'll get to in a moment...

"every Bill so presented to the President for his signature and for promulgation by him as a law shall be signed by the President not earlier than the fifth and not later than the seventh day after the date on which the Bill shall have been presented to him."

The language here is pretty strong - the law shall be signed, not 'may'. The President's hands are tied here; a law must be signed between five and seven days of him receiving it.

So what are the other instances where the President has discretion? There are three few specific exceptions:

• If the Seanad agrees, the President can sign a law before the fifth day, if it's deemed urgent. (This was the case when the overnight legislation was needed to liquidate IBRC, as there would have been a run on the bank if it was allowed to trade after the government had announced plans to shut it down.) The President has no discretion in this case.
• If the President believes a bill may be in conflict with the constitution, he can then convene the Council of State to discuss his concerns. If he then STILL has concerns, he can refer it to the Supreme Court for its binding ruling. If the Supreme Court says there is no conflict, the bill must then be signed into law straight away. If there is a conflict, the contentious sections of the bill are struck down.
• If a bill is passed by the Dáil but rejected by the Seanad, and the Dáil tries to power ahead with it anyway, the President can be presented with a petition asking him to hold a referendum on it before he signs it. However, that petition must be signed by at least half of the members of the Seanad, and a third of the members of the Dáil - and even then, he still has the discretion to ignore it if he so chooses. However, this clause can only be applied if the Dáil and the Seanad are in conflict - the President's role in this case is simply to break the deadlock between the houses by considering a referendum.

But those are the only exceptions.

Therefore, the only way in which President Higgins could have had some discretion not to sign the various laws on water charges would be if they were in potential breach of the Constitution.

That, as far as this reporter can recall, was not a concern raised by any opponents when the various laws were being passed through the Oireachtas.

Nobody denies protestors the right to vent their frustrations - that's the whole nature of a democracy. But it might serve the protestors, if they're going to demonstrate when the President tries to help a local school mark its 50th birthday, to make it clear exactly what they expected him to do.