I'm a little surprised that you're arguing anecdotally against a statistical generalization. But once more into the breach...

You're citing the IRA as a terrorism success? Let's look at that:

"The IRA's stated objective is to end "British rule in Ireland," and according to its constitution, it wants "to establish an Irish Socialist Republic, based on the Proclamation of 1916."[3] Until the 1998 Belfast Agreement, it sought to end Northern Ireland's status within the United Kingdom and bring about a united Ireland by force of arms and political persuasion."

Yeah, how's that worked out for them? Oh right, Northern Ireland still exists! How about that.

"In 1988, the PLO officially endorsed a two-state solution, with Israel and Palestine living side by side contingent on specific terms such as making East Jerusalem capital of the Palestinian state and giving Palestinians the right of return to land occupied by Palestinians prior to the 1948 and 1967 wars with Israel."

Funny thing, one never hears of any Palestinian legislation being passed in East Jerusalem, or any rights of returns. That's because, you know, the PLO accomplished jack squat.

"Hamas wants to create an Islamic state in the West Bank and the Gaza strip, a goal which combines Palestinian nationalism with Islamist objectives.[41] Hamas's 1988 charter calls for the replacement of Israel and the Palestinian Territories with an Islamic Palestinian state."

Israel is still there, and still killing as many Palestinians as it pleases. Your vaunted organizations haven't achieved a heck of a lot of their goals.

As for Hezbollah:

"Hezbollah's 1985 manifesto listed its three main goals as "putting an end to any colonialist entity" in Lebanon, bringing the Phalangists to justice for "the crimes they [had] perpetrated," and the establishment of an Islamic regime in Lebanon.[12][13] Recently, however, Hezbollah has made little mention of establishing an Islamic state, and forged alliances across religious lines.[11] Hezbollah leaders have also made numerous statements calling for the destruction of Israel, which they refer to as a "Zionist entity... built on lands wrested from their owners.""

I'd note that what notable successes they've had stem from guerrilla campaigns and open warfare; and not primarily terrorism.

Finally, I get the impression you didn't even bother to read the paper I carefully linked, Why Terrorism Does Not Work, specifically to address such objections. Let's spoonfeed some important bits...

"Not only is his sample of terrorist campaigns modest, but they targeted only a handful of countries: ten of the eleven campaigns analyzed were directed against the same three countries (Israel, Sri Lanka, and Turkey), with six of the campaigns directed against the same country (Israel).19 More important, Pape does not examine whether the terrorist campaigns achieved their core policy objectives. In his assessment of Palestinian terrorist campaigns, for example, he counts the limited withdrawals of the Israel Defense Forces from parts of the Gaza Strip and the West Bank in 1994 as two separate terrorist victories, ignoring the 167 percent increase in the number of Israeli settlers during this period—the most visible sign of Israeli occupation.20 Similarly, he counts as a victory the Israeli decision to release Hamas leader Sheik Ahmed Yassin from prison in October 1997, ignoring the hundreds of imprisonments and targeted assassinations of Palestinian terrorists throughout the Oslo “peace process.”21 Pape’s data therefore reveal only that select terrorist campaigns have occasionally scored tactical victories, not that terrorism is an effective strategy for groups to achieve their policy objectives."

"This study analyzes the political plights of twenty-eight terrorist groups—the complete list of foreign terrorist organizations (FTOs) as designated by the U.S. Department of State since 2001.7 The data yield two unexpected findings. First, the groups accomplished their forty-two policy objectives only 7 percent of the time."

"Using this list provides a check against selecting cases on the dependent variable, which would artificially inflate the success rate because the most well known policy outcomes involve terrorist victories (e.g., the U.S. withdrawal from southern Lebanon in 1984). Furthermore, because all of the terrorist groups have remained active since 2001, ample time has been allowed for each group to make progress on achieving its policy goals, thereby reducing the possibility of artificially deflating the success rate through too small a time frame. In fact, the terrorist groups have had significantly more time than five years to accomplish their policy objectives: the groups, on average, have been active since 1978; the majority has practiced terrorism since the 1960s and 1970s; and only four were established after 1990."

"As frequently noted, Hezbollah successfully coerced the multinational peacekeepers and Israelis from southern Lebanon in 1984 and 2000, and the Tamil Tigers won control over the northern and eastern coastal areas of Sri Lanka from 1990 on. In the aggregate, however, the terrorist groups achieved their main policy objectives only three out of forty-two times—a 7 percent success rate.31 Within the coercion literature, this rate of success is considered extremely low. It is substantially lower, for example, than even the success rate of economic sanctions, which are widely regarded as only minimally effective."

(Personally, I wouldn't even count the Tamil Tigers, as they currently seem to be screwed.)