No.  At worst, the loss of the IRC channel would delay the bootstrapping of a new client, but not prevent it.  Any client that has already bootstrapped successfully would not need access to IRC in any case.  I've personally tested this, because when I am at work, IRC is blocked by my employer's firewall.  The IRC channel provides quick access to peer discovery, but there is a built in default list of persistant clients to attempt a connection to if IRC isn't accessible, and once a client is bootstrapped it keeps a record of past connections and can search through those for viable peers upon restart.  Also, if IRC is blocked and the entire built-in list of peers is blocked or hacked, the client can be started with a switch to connect to any other peer that the IP address is known.  Which would permit existing users to post IP addresses for bootstrapping on any Internet forum such as this one, and create a set of moving targets for the attackers to pursue.