If you recently downloaded mIRC from the official mIRC webstite at http://www.mirc.com
and your anti-virus software is reporting that it contains a trojan/virus, this is likely due to a false-positive.
A false-positive is when anti-virus software makes a mistake and incorrectly thinks a file contains a trojan/virus when it does not. Unfortunately, this a common occurrence. For example, some years ago Microsoft Security Essentials detected the Google Chrome web browser as a trojan and recommended that users delete it. It can happen when an anti-virus company has not updated their signature database to include newly released software or has updated their database without checking it properly. The result is that the anti-virus software starts incorrectly detecting some files as trojans/viruses.
You can upload your copy of mIRC to https://www.virustotal.com
to check it. Note that, in general, there will always be some anti-virus applications that incorrectly report finding a virus/trojan. If the vast majority of anti-virus applications report no issues, that usually means that the file is safe.
You can also 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 signature database and should be able to tell you how to prevent their software from behaving this way in the meantime.
Some anti-virus applications are more sensitive to IRC applications than others and may actively block/delete their files. In such a case, you may need to add mIRC to the anti-virus application's exclusion list to get around this. If that does not work, you would need to contact the anti-virus company for help.