It tolerates partitions just fine; I've been through many netsplits where folks just kept talking on our side of the split until the network healed.

Partition tolerance doesn't mean partitions don't affect the system, or that they can't happen. It just means the system has to choose whether to become unavailable or inconsistent (since it can't have both in the presence of a partition). IRC chooses to remain available, at the cost of losing messages for people on the wrong side of the split.