I first encountered multi-user computing in the form of Swarthmore's VAX/VMS mainframe. While you were logged in, you could set something called your "process name", which would then show up next to your username whenever anyone did a SHOW USERS command to see who was on. People put all kinds of stuff there, from their real name to names of English country dances to moods to random words. I used to (and still do) make up passwords that sound vaguely word-like, but aren't, since it's easier to remember something like "alaphene" than something like "azknmfbtz". I started using similar things for these "process names" too, and one day came up with "irilyth". I liked the way it sounded, and ended up keeping it for a while.
When I got an account on the Suns in the CS department, I asked if I could use it as my username there. Being generally relaxed types, they said sure, and for my junior and senior years, <irilyth at cs dot swarthmore dot edu> was my primary e-mail address.
When I graduated from Swat, and started work at Mudd, I originally used irilyth as my username my Unix workstation there too. After fall semester of that year, though, I started using it only for personal mail, because the poor elderly easily-confused professors seemed to find it hard to remember, and I've kept it as a general personal username since then.
It has no particular significance other than that I like the way it sounds. No one calls me "irilyth" in person ever, nor ever should, really; it's just a computer username. It's pronounced roughly "ear-uh-lith", with an accent on the first syllable, when it's pronounced at all. (The first syllable is actually closer to the first syllable of "irridescent" than to "ear".)
I've found it very convenient over the years to have one account for work-related e-mail, and another for personal use. It helps maintain a formal distinction between business and pleasure, even when the two often seem closely related. Also, on any system where I get to choose my own username, it's extremely unlikely that someone else will have chosen irilyth -- things like "josh" and "jsmith" aren't nearly as likely to be free. It also makes it a little harder to be anonymous, since you can be pretty sure that any irilyth you find on any system is probably me; it also makes it a little easier for people to find me, but I'm not a real private person anyway, so hey.