I personally feel that mIRC is the best IRC client ever. Unfortunately, mIRC is not available for Linux. I tried IRC clients available for Ubuntu to find out something closest to or better than mIRC.
Xchat IRC client
It is easy to use but it is missing certain features that most mIRC users would find necessary. For example when you double click on a nick in a chatroom it displays whois info in the status tab instead of opening a query or private chat window. You can change this behavior in settings by replacing “QUOTE WHOIS %s %s” with “QUERY %s%s”. When you select some text in a chat window and right click it does not you show you an option to copy the selected text. You will have to use your keyboard shortcut CTRL+C to copy. In the input textarea, right click shows an option to paste. There are lots of other options in settings that you can change to make xchat act a lot like mIRC.
There is Xchat Gnome the default IRC client for Ubuntu. Since Dapper it doesn’t get installed by default you will have to install it yourself. This IRC client is the gnome interface for X-chat popular IRC client. What I personally don’t like about it is that it doesn’t have tabs. Windows are listed in a sidebar tree. It has plugins support but I personally found Xchat IRC’s settings a lot better than this Gnome client.
You can chat on IRC using Gaim but the problem wit Gaim is that it treats IRC the same way it treats Yahoo! or MSN chat. People coming from mIRC wouldn’t find it easy and comfortable. For example Nicknames in the chat window (not the user list) are not clickable. So if someone says something on the main of a chatroom and you want to Private Message them then you will have to either type the command or find their Nick in the userlist and then double click it.
I also tried LostIRC, a dead simple irc client but it lacks some really basic interface features, BitchX was too advance for me. I haven’t tried Chatzilla because it was a 10 MB download. I also didn’t try any IRC client for KDE but previously when I was testing Kubuntu, I tried Konversation and loved it. Right now I am using Xchat IRC client and would love to hear if there is something better than that.