Haven't gone too far with simplicity on my host, but I've been testing some software projects at suckless1 on my computer.

ii (irc it)
Besides from not supporting SSL/TLS on IRC (by default, and that's not a bug) it runs excellent when combined with tmux3 , I just use three tabs in the multiplexer (for each IRC server) , basically on this mode:
- ii running on the first tab
- a shell prompt where I can "echo" messages to the channel 'in' FIFO
- another prompt showing the output of 'tail -f $irc_server/$irc_channel/out'

I don't know if I'm doing it right but it works for me , lol
Though I'll keep using irssi as my main client, which already works fine, XD.

Only tested with just one channel on one network, as I still need to figure out how to do multiple channels on different servers.

2: <http://tools.suckless.org/ii>
3: <https://github.com/tmux/tmux>

st
It just works, just I haven't gotten used to it as my main terminal emulator, but it's otherwise fine. 
I guess for tab support either tabbed5 or even tmux / byobu[6] should work as well.
Never figured out how to use tabbed, even though I gave a read to its manual page.

Update #1: I'm using this as my main terminal emulator as of now
Update #2: I figured out how to make tabbed and st work side-by-side as of today

4: <http://st.suckless.org>
5: <http://tools.suckless.org/tabbed/>
6: <http://byobu.org/>

1: <http://suckless.org>