DAAGAAK  Sorry, this was not the problem. I can compile the fe-text front end no problem, however the MacIrssi GUI counts as a completely separate frontend (fe-aqua) and therefore fe-text is never compiled in alongside fe-text. This is why there is no /lastlog at the moment because the command itself is part of fe-text, not the Irssi core.  
(Version 0.8.6.6)

DAAGAAK  I'm sorry, /lastlog is a feature of the curses text frontend of Irssi which is not compiled into Irssi when it is compiled for inclusion in MacIrssi. (MacIrssi counts as a completely separate frontend). Adding a MacIrssi implementation isn't planned at the moment, if you file it in trac as a ticket I can try and plan it for future work  
(Version 0.8.6.6)

DAAGAAK  I appreciate it was annoying, possibly not as world ending as you implied though. It is now fixed in 0.8.6.5 onwards. If you find bugs like these, please feel free to file a ticket in my trac installation at http://trac.sysctl.co.uk/macirssi/.  
(Version 0.8.6.6)

DAAGAAK  I'm working on it, however it was more important to get a working Snow Leopard build and this necessitated the change in perl support.  
(Version 0.8.6.6)

DAAGAAK  There is also irssi config state stored in ~/.irssi/ which you will need to reset if you've managed to get it into this state. If you can reproduce the bug please head over to http://trac.sysctl.co.uk/macirssi to submit bugs.  
(Version 0.8.6.6)

DAAGAAK  MacIrssi only supports G4 or newer processors at the moment. Though I do concede that I don't mention that on the website.  
(Version 0.8.5.8)