is it true that someone can hack trough mIRC?

Technically, yes, but it's not as simple as it sounds and there have been a number of changes to mIRC to cut down on a person's ability to hack through mIRC.

Some of the preventative methods used are:
default disabling of auto-receive for high risk file extensions
default disabling $decode
Pop-up display when a script with ON START or ON LOAD events is loaded.

There's probably a lot more, but those are the items that I can think of right now.

Depends what you mean. There is no open doorway in mIRC through which anyone can just "hack" your system. If you run scripts or type commands which people give you without thoroughly knowing what they do then obviously people can do things to your computer, but it isn't possible without you running those commands/scripts first.

In other words you can only be hacked using mIRC if you don't use common sense.