redirect to specific pool based on host

As proposed by jgranieri, the easiest and best solution is to use Local traffic policies.

If you want to use irules, the switch command is better than if / then / elseif / elseif ...

when HTTP_REQUEST {
    switch [HTTP::host] {
        "linked.jan.nl" { pool p_sfb_a }
        "lyncdiscover.jan.nl" { pool p_sfb_b }
        "owas.jan.nl" { pool p_sfb_c }
        default { pool Pool_none }
    }
}