Irbe's always been one of my favorites, although you do seem to be overstating the length of time that Irbe's Sharks teams were God-awful.
http://www.hockeygoalies.org/bio/irbe.html
One thing I noted with Irbe is that his game-to-game consistency definitely improved as he aged. With the Sharks and Canucks, he was consistently less consistant (now there's a tongue twister) than the average NHL goaltender. With Carolina, he was consistently more consistent (using the "VAR" statistic in the link above).
How much of that is a function of the teams involved, I haven't studied.
Overall, just looking at save percentage, Irbe equates to a slightly below-average goaltender over his career (but a few hundred goals better than replacement level due to his longevity). His actual performance was probably better than that, since I can't imagine those Sharks teams were sound defensively. Having said that, if you look at the Sharks' goaltenders in those early years:
http://www.hockeygoalies.org/bio/nhl/sanjose.html
Irbe looks to have been deliberately sheltered by the coaching staff in San Jose.
In 1991-92, it looks like no goaltender really got favorable treatment (probably a function of the entire team being a mess).
In 1992-93,
Brian Hayward played only 18 games, but his average opponent was 0.47 goals better than average. Both Irbe (-0.10) and
Jeff Hackett (-0.11) played a below average opponent (on average). This is consistent with a team protecting younger goaltenders. Here's where it starts to get strange...
In 1993-94,
Jimmy Waite faced above-average (0.11) competition, with Irbe facing below-average (-0.08). This is particularly odd since Irbe played the vast majority of the games.
In 1994-95,
Wade Flaherty faced above-average (0.30) competition, with Irbe facing below average (-0.16).
In 1995-96, Irbe's strength of competition (0.04) was sandwiched between
Chris Terreri (-0.13) and Flaherty (0.18).
It's very peculiar in my mind. Some of this is explained by backups typically playing tougher competition than starters (since backups are more likely to relieve starters against tough opponents), but the gap is wider than that.