mIRC can be easily uninstalled through the Windows Control Panel, in the same way as other windows applications,
However, if you are unable to uninstall mIRC, you are seeing mIRC appear on startup or you have found mIRC on your computer without installing it yourself (which can happen if you visited an infected website or through email), odds are that you have managed to get one of the trojans that drop a modified version of mIRC as part of their payload. It is very important that you completely remove all the files included in the trojan from your computer. In some cases you may have to manually do some editing to ensure this.
where you will find information and resources to help you. Be sure to follow the advice there about researching whatever is found on your computer to ensure complete removal.
You should also run a couple of good, currently updated virus scanners. There some free online ones that can be better at detecting IRC related viruses/trojans. You can use Google to find some free online virus scanners