I need Irule for app to use only one node at a time

You could create a separate pool configured with Priority Activation Groups in an Active/Standby configuration that you do not tie directly to the Virtual Server. If you have other traffic hitting this Virtual Server you can create a different default pool for them.

You can then use an iRule to redirect / filter the traffic that requires the special pool with an iRule.

You could do it based on the Host or the URI:

 
BY HOST:
when HTTP_REQUEST {
  if { ([HTTP::host] equals "www.myspecialdomain") } {
pool specially.configured.pool
  }
}

BY URI:
when HTTP_REQUEST {
  if { ([HTTP::uri] starts_with "/specialuri") } {
pool specially.configured.pool
  }
}