Irule URL matching issue.

Based off of the code snippet you've provided I don't see why you would need the -glob option, but I don't think that is causing your issue. With the way you have this configured, I don't see any way that dev.mysite.com would match your first case.

Do you have a default pool defined on your virtual server, and if so, is it pool 1? Maybe you're not matching either of your cases in your switch statement and you are just going to the default pool.

If I were you, I would put some log statements in to determine the path you are taking. Something like this:

when HTTP_REQUEST {
    log local0. "Host header is [HTTP::host]"
    switch  [string tolower [HTTP::host]] {
        "admindev.mysite.com" {
            log local0. "matched admindev.mysite.com"
            pool number1
            } 
        "dev.mysite.com" {
            log local0. "matched dev.mysite.com"
            pool number2
        }
    }
}