How many threats did actually turn out to be real to date? I couldn't find this being published. But how many threats did happen without any indication (only after the perpetrators tell). I can easily recalled maybe 3-4 incidents. So the issue here is do knowing threats really help?

There was literally no threat.

In the simplest possible terms: this is total bullshit security theatre. At no point has there ever been a bomb or even a bomb threat carried out via usb device names. There is absolutely no reason to even look at the names of Bluetooth devices on a flight.

The PIRA actually do seem to have intended to give accurate warnings when they planted bombs, in Belfast at least. There were inevitably cases when the information was garbled or misunderstood but the use of codewords & the practice of delivering the warnings to a known set of media outlets was at least an attempt to minimise these.

The downside was that the vast majority of warnings were hoaxes - bomb scares were dozens of times more common than actual bombs.

The other main groups - INLA, UVF, and UFF/UDA also got in on the hoax game, but didn't often do real bombs (and didn't always give proper warnings when they did - see the UVF's Dublin & Monaghan bombings for a particularly grim example).

But real bombs were just common enough that the hoaxes from whatever source had to be taken seriously and so they caused huge amounts of disruption, probably more than anything that actually exploded.