Flash Player can't publish an IP multicast. only Flash Media Server 4 (Interactive or Enterprise) can publish an IP multicast. you can download the developer version for free; however, IP multicasts will be limited in duration to (i think) 10 minutes.
you mentioned doing ip Multicast "over LAN without Cirrus" which might be native IP multicast and can be done using the following when setting up the group
var groupspec:GroupSpecifier=new GroupSpecifier("your_group_name");
IP multicast is used to discover peers in that case, but Flash Player can't publish a stream over IP multicast (only P2P).
if P2P is disabled, peers can't join together into a group even if they try to use IP multicast to find each other on the LAN. and with no peers and no IP multicast stream source, the stream won't flow.