There cant be many "differences" in how they function because they both have to do the same "jobs", i.e. ON TEXT for example, there has to be the same thing in TCL. however, saying that.
BIND in TCL supports *everything* where as mIRC has different events, ON TEXT, NOTICE, ect.
I'm not saying it would be impossible to convert it, what i am saying is that i doubt anyone would create such a thing since there is no need really.
Also, yes it is "very possible to compile a mIRC Script into TCL code", but then, when isnt it?, its very possible to convert VC++ to VB, or, PHP to ASP, unless the language you are converting into has some shortfalls, for example, you cannot convert a mIRC script that has sockets in it to TCL, if TCL didnt support sockets (which is does).