There are a couple of methods you could use to automatically join channels when you connect to IRC.
Firstly, mIRC has a built in 'Perform' section in it's options - this section will automatically perform commands when you connect. To access it, go to:ALT+O > Connect > Options > [Perform...]
In the white box put the /join command and then check the box at the top so Perform is enabled. To join multiple channels, specify /join #channel,#channel2,#channel3
and so on.
You can also do this via scripting although it's unnecessary due to the Perform section. You may like to see this thread
for som examples.
Hope this helps.