You can log onto more then one server at a time with Mirc.
Most people just idle in IRC anyway, and you can idle in several servers at a time if you really need to get a trade done, this is just another way to do that. Or if you are just browsing for the heck of it, you can stick to your favorite server....
As for the MAC, here is a google search that brought up several IRC clients for me on the MAC. Not sure on how good they work, but they are out there. My disclaimer is that these may or may not be easy to set up and use, not anything like MIRC in other words.
Search terms: Mac download IRC client
Never used a Mac, so someone else may have to post on what any of those are like.