I could try to answer your questions on the IRA, but I am much more qualified to address those same questions with regard to ETA, so here it goes:

ETA undertook a number of successful operations, arguably the most important was Ogro Operazioa (Operation Ogre) which resulted in the death of the recently-appointed Prime Minister and Franco's heir apparent, Luis Carrero Blanco. I'll use this as an example of how ETA was able to pull off an operation of this calibre.

In 1972, ETA's central committee sent a commando (4 men) to Madrid (ETA's first time operating outside of the Basque Country) to study Carrero Blanco's everyday movements and develop a plan to kidnap him. The idea was that he would be used as leverage in negotiations to free a large number of ETA prisoners as well as publicly broadcast ETA propaganda. But when Carrero Blanco was promoted to Prime Minister in 1973, his improved security detail made a kidnapping impossible. The commando returned for briefing in the Basque Country in the summer of 1973, where they received their new directive: they were simply to assassinate him.

Let's pause to seriously consider the logistics of such an operation. ETA had to pay for the upkeep of 4 people living in Madrid for a whole year while they planned and eventually carried out Operation Ogre. This wasn't inconceivable. The 4 etarras rented a fairly affordable apartment together, drank sparingly, lived off cheap sandwiches, you get the idea. Anyway, the apartment had a basement suite adjacent to the street where Carrero Blanco frequently drove over on his way to church. The 4 young ETA members told their landlord they were art students working on sculptures as cover for the noise they made digging a tunnel from their basement underneath the road to fill with explosives. After weeks of digging, with ill-fitting tools and guided by a book on mine construction bought at a local shop, they had built a 21-foot long tunnel that crossed underneath the road. ETA's central committee delivered to them a vehicle packed with over a hundred kilograms of Goma-2 explosives which they had stolen a year earlier in a raid on a powder magazine in the Basque Country. They packed the tunnel with explosives, hooked it up with an above-ground detonator, and on December 20th 1973 they killed Carrero Blanco (and his two bodyguards) as he drove over. They laid low at a pre-arranged safehouse for a few days before returning to the Basque Country.

So what can we conclude from this? The whole thing was surprisingly amateurish. 4 men in their early to mid-twenties were able to blow up the apparent successor to Francisco Franco, and did so with shockingly little experience and equipment. Reading their account of Operation Ogre is remarkable for its banality. There are several other operations/attacks I could discuss but as a general answer to your question I believe you would find that many of these 'devastating attacks' were shockingly easy to perpetrate for ETA. It was not until many years later after the fall of the Franco Regime when national security and intelligence vastly improved that ETA repeatedly faced difficulty in carrying out operations in Spain. I've simplified several things herein for the sake of brevity so if you have any more questions feel free to leave a comment or message.

All the details of the operation can be found in the primary source: Julen Agirre (pseudonym), Operación Ogro, 1974.

For further reading on the internal workings of ETA, I would recommend:

Javier Vinader, Operación Lobo, 1999.

Xabier Zumalde, Las botas de la guerrilla, 2004.

And above all, though the most difficult to obtain, a multi-volume compilation of ETA primary source documents simply titled Documentos published by Hordago in the late 1970s.