There's no 'on echo' event and no scriptable way to catch that kind of thing properly. You'd have to specify precisely how you're sending data to mIRC in order for anyone to really tell you the best way to capture it.
As for evaluation, it's simply a case of using $eval(%variablename, 2), however be aware that this is potentially very dangerous unless you're certain the data being evaluated is always 100% trustworthy.
