ISL on Fiber Channel networks has nothing to do with the _ISL trunking protocol_ known on Ethernet networks. In Fiber Channel networks, it is simply a way to describe the connection between two Fiber Channel switches.
In Ethernet networks it's a trunking protocol tagging frames with the appropriate VLAN information (as 802.1Q does, which is actually the standard). ISL is a Cisco proprietary trunking protocol pre-dating 802.1Q; it's deprecated and is already removed from the newest switches.