Statistical analysis of terrorist groups' longevity, aims, methods and successes reveal that groups are self-contradictory and self-sabotaging, generally ineffective; common stereotypes like terrorists being poor or ultra-skilled are false. Superficially appealing counter-examples are discussed and rejected. Data on motivations and the dissolution of terrorist groups are brought into play and the surprising conclusion reached: terrorism is a form of socialization or status-seeking.
http://www.gwern.net/Terrorism%20is%20not%20about%20Terror
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:
Yeah, how's that worked out for them? Oh right, Northern Ireland still exists! How about that.
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.
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:
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...
(Personally, I wouldn't even count the Tamil Tigers, as they currently seem to be screwed.)
Already read that paper. It uses the unrealistic criteria of "achieving objectives". Nobody achieves their objectives. The Republicans were in control of the US for 8 years and didn't achieve their objectives. What percentage of US Presidents "achieved their objectives"? Less than 7%, I'll bet.
England made compromises with the IRA. Israel has made compromises with the Palestinians. This would not have happened without terrorism.