THE RELEASE of a hostage is always an occasion of mixed emotions: joy for the one released, anger at the continuing plight of the others and, often, at the tales the newly returned hostage has to tell. The release of Irish hostage Brian Keenan after 4 1/2 years in captivity was just such an occasion; but Mr. Keenan, through his extended and strikingly eloquent remarks on his captivity, made it into something more stirring and instructive. As the years drag on for the Lebanon hostages, and as events in the region add layers of complication to their plight, the world sometimes has a struggle to keep them sharply in mind -- and this despite the dedicated efforts of the friends and families of those being held. Who in normal life, after all, can imagine what it would be like to be a hostage? Into this murk, Mr. Keenan casts a powerful light.

" 'Hostage,' " he told journalists in Dublin, "is crucifying aloneness; there's a silent screaming slide into the bowels of despair. 'Hostage' is a man hanging by his fingernails over the edge of chaos, and feeling his fingers slowly straightening. ... We all often thought, how could we relate to anyone, how could we relate to anyone what we had been thinking? Tiny, tiny cells, constant blindfolds, prolonged days in the dark, sometimes weeks without light create kinds of insanity that drive men deep, deep into themselves."

In his 40-minute press conference, Mr. Keenan talked a great deal about his fellow hostages and their fortitude, the games and crazy projects they resorted to in order to stay sane -- lectures, imaginary characters, even comic imitations of the guards who kept them in line -- and about the difficulty of hanging on as the time lengthened. Courage, he suggested, "is not something that's got to do with physical strength or aggression, courage is something that has to do with the ability to control fear. My fear has different depths and different qualities. Some of it is beyond any man's ability to control fully." How, he asked, "can you control the mind when the mind itself has already chosen to go its own way, independent of one's will? ... Each of us fathomed the depths of mania, of despair, that awful howling wilderness where one felt like no more than a speck of dust in an alien cosmos."

Mr. Keenan told vivid and affectionate stories of his fellow hostages, especially his friend John McCarthy, still in captivity, with whom he was held and with whom he became close. That testimony made a friend of Mr. McCarthy's smile with visible relief. The darker parts of Mr. Keenan's story, more painful to listen to, have at least as great a value. His words will make it all the harder for the rest of us to forget his comrades who are still hostage.