Monospacing is an actual "font".
I dont believe it allows any font to use it.

It is only used when alignment is "critical" to ensure all the letters had the same width, which wouldnt be the case in mIRC it would just be a personal preference as there is nothing 'critical' about the width of a font.

If you wanted Monospacing, simply used FixedSys, most other fonts although not monospaced, you dont realistically notice the difference until 'i' and 'w' are typed.

Eamonn.