is this a sin?
It would depend on the reasons - I come from a nation where the hunger striker has been used historically for numerous reasons. I would say in those cases it was not a sin as the intent was not to commit suicide.
There is certainly some space within which depriving oneself of adequate nourishment can be morally permissible - if some form of even physically deleterious “fasting” is necessary for one’s spiritual good (although I couldn’t give you any examples approaching the drastic nature of a hunger strike where I imagine that could actually be the case), then at least in principle it would be okay. Hunger strikers, though, harm themselves for the purposes of amending another person, an aim which has no certain connection to the chosen physical deprivation in either the physical or spiritual orders. Accordingly, no principle of double effect can be invoked. Were someone to die as a result of persistence in a hunger strike, it would be the sin of suicide. Even if they stopped before dying, it would seem that any outright harm they had done to themselves through the strike would be in violation of the 5th commandment.
The Church has never condemned the numerous hunger strikers in my own nation as suicides and many of them were buried in hallowed ground. Several of them were highly devout Catholics, see for example the gentleman below:-
en.wikipedia.org/wiki/Terence_MacSwiney - note that MacSwiney’s remains were on display in London openly in a Cathedral and he was also attended by a priest throughout his time on hunger strike.
Even more modern hunger strikers who have been far less devout have not been condemned as suicides.
With specific reference to the IRA hunger strikers, the Irish bishops conceded:
‘‘there is some dispute about whether or not political hunger striking is suicide or more precisely, about the circumstances in which it is suicide.’’
But the Americans took a clearer line:
“Referring to the position among Catholic moralists, the Rev. John Connery, a theologian with the United States Catholic Conference of Bishops, said: ‘‘The common opinion is that refusal to eat and trying to achieve a purpose by death is wrong and would have to be classified as suicide.’’
Cardinal Cooke suggested that, within the context of compassion, Mr. Sands’s death was nonetheless suicide. ‘‘I beseech God, who loves every human person,’’ the Cardinal said, ‘‘to enlighten our minds with the realization that peace cannot be established by violence, even by the violent taking of one’s own life.’’
At the time of the death of Mr. Sands, Archbishop John Roach of Minneapolis, president of the bishops’ conference, wrote to Cardinal O Fiaich, expressing ‘‘profound regret at the tragic death of Bobby Sands despite the efforts of the Holy Father and yourself and others to avert this useless sacrifice.’’”
This was the sacrifice of a soldier in Sand’s case. Also the two gentlemen I mentioned previously lived 60 years before Sands and Ashe and MacSwiney were both devoutly Catholic. The latter’s funeral mass was said by a Bishop.
Here is what Cardinal O’Fiach had to say concerning the death of one hunger strke:-
“Raymond McCreesh was captured bearing arms at the age of 19 and sentenced to 14 years’ imprisonment. I have no doubt that he would have never seen the inside of a jail but for the abnormal political situation. Who is entitled to label him a murderer or a suicide?”
I quite agree.
His Eminence’s comments seem a bit more informed by patriotism than theological rigor, as do the other justifications for hunger-striking I have seen. On article by a Jesuit written near the time of MacSwiney’s death is a very solid attempt, engaging the authorities of Suarez, Victoria, Ligouri, and the other great (early-)modern moralists. But while it manages to defend its assertion that refusing basic nutrition can sometimes be morally permissible, it makes no effort to address other equally important circumstantial elements, such as proportionality, chance of success, or last resort. Instead the author seems to rely on what he presumes to be our shared moral outrage at the British occupation which should undoubtedly lead us all to conclude that you can’t go wrong if you’ve done something to oppose the bloody Brits. Given, then, that even the Irish bishops of the modern day could offer no more than their uncertainty (“there is some dispute”) about hunger striking and this uncertainty stands opposed to the common opinion, I should think that the burden of proof for overturning the common opinion should require more than the actions of a single bishop.
"I am confident that my death will do more to smash the British Empire than my release.
He was quite right, he was a hero of our nation and I salute his gallantry.