I've never actually looked into SILC. What is it, exactly? What benefits are there to using SILC over IRC?
I'm content with using IRC, and would/will probably switch if/when SILC gains some momentum and people start moving over to it.
It's designed for (cryptographic) security through the entire protocol. Users can be positively identified via public key cryptography (assuming they choose to use the same public key while connecting) and all communications are encrypted with the server serving as a relay point after a key exchange with it (requiring that, like ssh, you can verify the server's public key). There are also provisions (such as user to user or user to channel client to client encryption) to ensure that a compromised server or evil server operator cannot eavesdrop, if you choose to use those functions.
It's also, in general, a much better thought out protocol than IRC (among other things, not reliant on parsing human-readable text messages everywhere). Unlike IRC over SSL, the security features aren't an afterthought bolt-on either.
The end user experience is fairly comparable to IRC.
