For instance would the Irish people have their independence from the UK if there was no IRA? If there was no violence why would the UK give them something they;re not willing to take, especially when states typically only respond to violence.
Depends which movement called the IRA we're talking about and (more generally) what's meant by "terrorism". There's a degree of a difference between the tactical approach of IRA that won the independence of the Irish Free State in the early twentieth century through classic guerrilla warfare tactics aimed at weakening British security forces and making Ireland ungovernable and and some of the more stereotypical terrorist activities carried out by the Provisional IRA in the late twentieth century involving the high profile bombing of city centres with the (wholly unsuccessful) aim of weakening the British public's resolve to continue to support the Union in Northern Ireland.
There were also immediate wins, particularly dismantling the RUC which performed attacks against civilians and colluded with unionist terror groups.
(disclaimer: I believe attacking civilians is awful and god isn't real)
They could have had something similar to the Good Friday Agreement much earlier had they been willing to pursue that type of settlement by political means earlier.
I imagine that this wasn't one of the goals of the IRA, but it looks to be a major motivator for ISIS, which is trying to recruit from the vast majority of peaceful Muslims in the world with a narrative of West-vs-Islam. If even a very small number of Muslims are radicalised because their mosque was burned down or their mother assaulted, for example, that's still a win for the terrorists.
Due to the asymmetric nature of terrorism, which after all is the whole motivation behind using terrorism in the first place, I think it's useful to apply some of the same thinking we do when considering the payout for spam email - if one in 100,000 buys the product / falls for the scam, it's a success.
Daesh believes the ends of times are coming and the final battle against "evil" is near so they are just doing Allah's bidding, they believe nothing has changed since the 8th century and they are fighting "the armies of Rome". Do you really think it is rational thinking ? do you really think their can be some kind of political solution in that case ? Daesh is forcing no change upon the enemy, they want to exterminate them like Nazis wanted to exterminate the Jews.