It's ironical that Linux, the most secure OS, is commonly used to hack other machines.
Is that sentence correct, with respect to the irony part?
I don't see what's ironic about it, honestly. What is it about a more secure system being used to hack a less secure system that violates any expectation? That's like saying it's ironic that the reliable, well-armored, heavily armed German Panzer III tanks beat the bejesus out of the legendarily crappy British A9s and A10s in World War II. Well, no, that's really just pretty much what you'd expect to happen.
The definition of irony can be so elusive. It may help to think of irony as the juxtaposition of two contradicting states, one which is obvious and one which is unexpected.
Let's examine the two pieces of your example:
- Linux is the most secure OS.
- Linux is used to hack other operating systems.
If we could state that the Linux development community put a high value on developing a secure operating system in order to prevent hacking, then I think yes, we could consider this statement ironic.
In other words, we would be presented with two contradicting states:
- Linux is the most secure OS because Linus Torvalds wanted to prevent hacking.
- Linux is used for the most effective hacking.
Change "ironical" to "ironic" and you are spot on the money.