Hey Quantum,
Yeah, I am a freelance sound designer (at the moment!). So I have to spend stupid amounts of time working with these mad programs, figuring out how to get them sounding their best. Convolution is actually a technique that we were using in Csound ages ago, making weird filter effects. But more recently, developers have been using convolution to create realistic reverberations.
Essentially, the IR in this context is 'response' of an acoustic space to a sudden impulse. Usually they are made using one of two techniques - with a 'spike' (like a clap, or a gunshot) or a 'sweep' (usually a sine waves gradually swept through the auditory range). The resulting recording is then 'deconvolved' using a computer program.
Anyway, the IR contains the acoustic character of an acoustic space. The sweet thing is that this doesnt have to be a real one, you can record IR's from artifical reverb processors as well. So many people sample IR's from expensive hardware processors, check out this link
https://www.noisevault.com/index.php.
SIR is the standard freeware convoverb. I think it sound perfectly adequate although I know some people who think it lacks something of the commerical versions. I like Voxengo's Pristine Space. This one is cheap, sounds great and has plenty of functionality. Waves IR1 is also good, and comes with loads of free IR's. Altiverb for the Mac also has plenty of free IR's, and the library is constanty growing.
To be honest though, SIR is great and for free you cant grumble. Maybe the FFT algorithms arent QUITE a good as though in the Waves and Altiverb, but its really all down to the quality of the IR's. Thats the real deciding factor.
Btw, if you have Sound Forge you can do convolution in that. Its possible that Audicity can do it aswell, and that editor is freeware (and superb!).
Sorry, I am a total geek for this kind of stuff. Dont get me started...

SJ
PS. You dont need formal training to be a sound engineer. Its all about experience. A little techy knowledge is required, but not much. Knowing the instrument, acoustic space and recording equipment is the main thing. If you can get a good sound, then thats all that matters. Judging by your own recordings, you certainly have the knack as they sound very good to my ears.
Do a search on Google and you'll find loads of info on audio engineering. The 'Sound on Sound' site is a good resource as they archieve LOADS of old articles covering everything from mic placement to DSP processing.