Which anti-virus software are you using?
If you have recently downloaded the latest version of mIRC from the official mIRC webstite and installed it on your system, this is could be due to a false-positive which usually happens when an anti-virus company updates their virus definition files without checking them properly. The result is that the anti-virus software starts incorrectly detecting some applications or files as trojans or viruses. Unfortunately this happens all the time. For example, some years ago Microsoft Security Essentials detected the Google Chrome web browser as a trojan and recommended that users delete it. You would need to contact your anti-virus software company to report the issue and to ask them for a solution. They should then correct the error in the next update of their virus definition files and should be able to tell you how to prevent their software from behaving this way in the meantime.
In addition, some anti-virus applications are more sensitive to IRC applications than others and will actively block/delete their files. You would need to add mIRC to their exclusion list to get around this. If that does not work, you would need to contact the anti-virus company for help.
You could also try uploading your mirc.exe to https://www.virustotal.com/gui/home/upload
to check it.