Is there a way to use the 'merge log' feature from outside of mIRC?
Is there a way to merge those logs automatically at the end of the day (from within mIRC)?

You can use DOS commands to copy them, but mIRC can also copy files into each other with a bit of scripting.

DOS sounds faster, just use the format...

COPY logfile1 + logfile2 + logfile3 outputfile

Use the + between each file to add, and end with your output file just a space separating the last file to add.

You should be able to use /RUN to execute the copy commands. Failing that you can use /write to create a batch file and /run the .bat file.

If you want to create a script, check the following help section with: "/help file and directory identifiers"

Good luck

In mIRC the command would be:

• copy -o in1.log out.log [color:gray]// overwrite output file
  copy -a in2.log out.log // append to output file
  copy -a in3.log out.log[/color]