Part of it is undoubtedly that the IRA had a lucrative criminal network in their heyday, and some people are bound to be reluctant to lose all that lovely money. But random assassinations don't further that: as any mobster could tell you, the last thing you want to do is cause needless trouble and draw the attention of the law.
A fanatic is defined as one who redoubles his efforts when his cause is lost, and this is a horribly clear example. Ireland probably will be reunited one day simply as a casual piece of administrative convenience, when the population is sufficiently integrated that nobody cares, and the factions of the Troubles are as much of a historical curiosity as Lancastrians versus Yorkists in England. But it won't be reunited under Catholic Sharia because that is passing away in the Republic, and the irridentist ex-IRAs, like militant Islamists in the Middle East, are fighting for something that no longer exists.