In The State (Lynch) v Cooney  IR 337, the Supreme Court upheld the validity of a statutory provision [section 31(1) of the Broadcasting (Authority) Act, 1960 (also here) as amended by section 16 of the Broadcasting Authority (Amendment) Act, 1976 (also here) – thankfully repealed in 2001] which allowed the Minister to preclude from broadcast any matter which “would be likely to promote, or incite to, crime or would tend to undermine the authority of the State”. O’Higgins CJ for the Court held that the free speech guarantee [Article 40.6.1(i)] of the Constitution
enables the State, in certain instances, to control these rights and freedoms. The basis for any attempt at control must be, according to the Constitution, the overriding considerations of public order and morality. The constitutional provision in question refers to organs of public opinion and these must be held to include television as well as radio. It places upon the State the obligation to ensure that these organs of public opinion shall not be used to undermine public order or public morality or the authority of the State. It follows that the use of such organs of opinion for the purpose of securing or advocating support for organisations which seek by violence to overthrow the State or its institutions is a use which is prohibited by the Constitution. Therefore, it is clearly the duty of the State to intervene to prevent broadcasts on radio or television which are aimed at such a result or which in any way would be likely to have the effect of promoting or inciting to crime or endangering the authority of the State.
A subsequent challenge to the section on the grounds that it was contrary to Article 10 of the European Convention on Human Rights was rejected in 15404/89 Purcell v Ireland (Decision of 16 April 1991). Moreover, the House of Lords in R v Secretary of State for the Home Department, ex p Brind  1 AC 696,  UKHL 4 (07 February 1991) sustained a similar ban; and an Article 10 challenge subsequently shared Purcell‘s fate (see 8714/91 Brind v UK (Decision of 9 May 1994)).
I was reminded of this today reading an article by Joanne Mariner (Terrorism and Counterterrorism Program Director at Human Rights Watch (HRW)) on Findlaw‘s Writ column. Her article is called “Terrorism and Speech”. Some extracts:
Restrictions of all sorts have multiplied in the heightened security environment of the last six-and-a-half years, so it should be no surprise that, around the world, legal restrictions on speech have tightened. Since 2001, there has been a clear trend toward prohibiting speech perceived as supporting terrorism, and toward barring the dissemination of materials–including books, videos, and other forms of written and graphic communication–that are believed to be of use for terrorist activity. … the legal trend globally is not only to criminalize direct incitement to terrorist activity, but to criminalize indirect incitement–to prohibit speech perceived as justifying, defending, or “glorifying” terrorism. This, from the standpoint of free expression, is problematic. …
The reasoning behind such laws is understandable. Governments want to stop terrorism before it occurs; indeed, they would prefer to deal with the problem before the potential terrorist gets anywhere near the stage of actually planning violent acts. Some proportion of the people who communicate support for terrorism, or who read terrorist publications, may one day be moved to action.
Still, a spate of recent prosecutions in the UK does little to instill confidence in these laws. Defendants have included a couple of 17-year-olds, and a young woman known (for her poems) as the “lyrical terrorist.” … By wasting scarce legal and prosecutorial resources going after speech, rather than action, governments may be doing more harm than good. The defendants in such cases no doubt see them as political and religious persecution, and their families, neighbors and larger communities may agree.