I am afraid the Mr. Hitchens may have been led astray, even if by so esteemed a source as the Oxford English Dictionary. Perhaps he picked up a very old edition. At any rate, the description of various armed groups, revolutionaries, terrorists, etc. as "dissidents" is not uncommon since the end of World War II. Hitch may want to believe that there has been some kind of agreement that the term dissident refers to "only attitudes, not actions", but that does not seem to be the case, OED or not.

Take a look in Hansard (the record of Parliament). There in 1946 you will find the term "dissident bodies" used to describe what are elsewhere called "terrorist groups" such as the Stern Gang and Irgun. In 1950 there was the prospect of fighting "dissident forces" in Fiji. In July of '55 there was talk of Aden "disturbances" being caused by armed "dissident tribesmen." In 1957 Mr. John Profumo (!) defended the British bombing of Danaba, Aden, on the grounds that "dissident tribesmen" had attacked an English patrol. In '56 armed Africans were said to be carrying out "active dissidence." In July of '57 there was concern that "modern arms" were reaching "dissidents" in Oman. In May of '58 there was concern that US arms were reaching "dissident forces" in Indonesia. By 1963 "dissidents" in the Congo were wreaking havoc. And so it goes....

None of the noted groups was pacifist, or even peaceful. They were clearly not just engaging in theoretical non-belief in majority views --- they were killing people and blowing things up. And they were being called "dissidents" by British leaders (MPs and Government Officials) of various stripes long before the recent application of the term to an IRA subgroup.

Is "dissident" such a bad descriptor? It is not mutually exclusive to "terrorist". Just because a newspaper uses only one descriptor, that does not mean that no other descriptors apply. Once everyone learns that the IRA is a "terrorist" group then that descriptor becomes redundant. It might be more informative to call them a "dissident" IRA group to emphasize that they disagree with the majority view of the IRA --- not that they are pacifist dissenters. After all, if all IRA are terrorists then some descriptor other than terrorist must be come up with differentiate sub-groups.

Times change, and language changes with it. Words that can be used as euphemisms are sometimes also valuable or have specialized meanings (within, say, political science). For example, most of what we today call "terrorist acts" were once called "outrages" by the British. But you can be outraged by a lot of things, and over time terrorism came to imply a political goal behind the violence and thus be a more specific descriptor of action by political groups (as opposed to ordinary "outrages" by hungry mobs, etc.). And just as one man's terrorist can be another man's freedom fighter, even the 'negative' connotations of the words themselves can be in dispute. In the 1970s there were vocal Jews in Israel who ranted against calling Palestinians "terrorists" because the word had a positive connotation, at least to those who had engaged in or supported Zionist terrorism. For example, a book title like "The Lady was a Terrorist" was considered laudatory in 50s Israel. Look at the evolution of -- and battles over -- the adjectives "conservative" and "liberal" in political discourse. The two words twist about one another like DNA over the course of centuries. Who is to say precisely by whom (or when) the correct definition was applied?