just registered to clear up a misconception you might be having. Blowfish IRC encryption is not comparable to the SSL encryption mentioned previously in this thread. SSL is solely client to server encryption whereas blowfish is used to encrypt the communication in channels, queries , notices and topics. The difference is quite obvious. Blowfish will make sure that communication between two IRC users is encrypted. Data sent via SSL server connection will be visible in plaintext on the server.
This also makes your question regarding the networks that use blowfish pointless. It's used on each and every major IRC network. I know a bunch of channels and users on DalNet, LinkNet, EfNet and several private IRC networks that use blowfish.
Due to the fact that blowfish encryption is built on top of the IRC protocol (as mentioned earlier) it is available on all IRC networks even if they do not offer SSL connections (on top of the extra security offered by the client to client pattern).
Personally i've been using blowfish on IRC since several years and know at least a couple of hundred people and channels spread out over several networks. The audience for this is larger than you might think. I am also assuming that a lot people would like that extra privacy who do not know of this method yet due to the fact that it's not readily available. When a new mIRC version is released i was forced to wait with upgrading until the blowfish plugins/patches/scripts got updated for new versions as well. Also there have always been rumors about malware/spyware/viruses in these dll's and patches, which leads to another problem with third party assemblies: the source isn't always trustable.
All the aforementioned are reasons why i would love a native integration into the mIRC core too. I think the implementation is rather trivial and the overhead and increased size generated by it is ignorable. Hope you and the team reconsider this feature request.