A converter from mIRC to Tcl would be an insane project.
Mainly due to its size. SOme of the mIRC command have switches that totaly change the way a command acts, and some have items that are not supported by Tcl (which wouldn't normaly be needed in Tcl anyhow).
mIRC is a lazy parser, that has no constant structure, and changes the way it works as it sees fit. Many would say the same with Tcl, which is not true.
Tcl defines a structure, and some commands, to extent the structure base.
In mIRC, the commands are rather slapdash. To prove a point, look at the timer command.
In stead of naming or giving it a id using a aparamter, you pass it on to the timer command itself.
The timer command would take a hell of a lot of work, as you couldn't just parse it out the normal way. Aditionaly, you would have to track the name/id so to know when to kill, pause or resume the timer.
This would likely involve a complex wrapper function, that allows you to assign your own ID/name to the timer, seeing as u/timer pass there own names.
Variables would also be a sore point. Every variable would have to become global unless the script parses the "var" command.
Parsing "ON" events would be tough too. You would need to account for the fact, mIRC uses one event for msg and pub binds.
Most of these points, are likely to be the reason for why this has never been done. Don't get me wrong, the need/requirment has allways been there, but it is such a large project.