mIRC already communicates with the server via ping pong events to determine if you are still connected, and will disconnect you are an amount of time goes by with no reply's (much less than the 20-30 minutes you are experiencing). The server side however has its own timeout period before the nick is killed for timing out.
The exception i have noticed with this behavior is when using ssl, from time to time i also seem to be connected and attempt to send a message (which fails and then finally disconnects me), while infact i had timed out from the server hours ago sometimes even more. However i have only ever experienced this issue when connecting with ssl.
As stated, you could script custom ping pong events or anything really to check your connection is still active, but be aware if there is server lag you could incorrectly disconnect yourself periodically. Also, sometimes there are 'pauses' in your internet connection that cause things to stop responding, however when the activity resumes your connections will resume without disconnection, by scripting something you would likely force yourself to disconnect in these circumstances also.
