He passed away earlier this week. He was 32

That's depressing. I was really, really hoping that Doyle would eventually return.

Does anyone know how he died? I have yet to see a source that says.

The LA Times said this:

The Dublin-born Quinn was found dead from a possible drug overdose at a friend's home, Los Angeles police said Friday. Detectives said the case will remain open pending the completion of toxicology tests, but no foul play was suspected.

Here's Entertainment Weekly's tribute to Glenn:

GLENN QUINN MAY 28, 1970-DEC. 3, 2002

''It seems like everything's coming together this year,'' the Dublin-raised actor told EW in 1992, when he was juggling roles on two ABC series, Roseanne (as none-too-bright son-in-law Mark Healy) and the short-lived Covington Cross. He appeared on Roseanne through its 1997 finale, then won a cult following as Doyle on The WB's Angel. His character was written out in 2000 after nine episodes, and an impassioned online campaign for his return proved in vain. (Quinn was found dead of an apparent drug overdose in L.A.)

His bios on the Season One DVD set doesn't mention that he died, instead it says he's living in California.

Oooops!