Could someone help me with a simple redirect (iRule)?

You might consider using a Traffic policy (if you are on TMOS 11.4 or greater) 2 Minute Tech Tip: URL Redirects

If not then you might want to consider an iRule something like this:

when HTTP_REQUEST {
     if { [string tolower [HTTP::path]] starts_with "/admin" } { 
     HTTP::redirect "/"
     }
     }

Either way you will have to apply the traffic policy to the HTTP and HTTPS virtual server.