First start irssi and connect to the network you would like to autoconnect in the future, register your nickname with nickserv and then type in the following:
/server add -auto -network fn irc.freenode.net /network add -nick user -autosendcmd "/msg nickserv IDENTIFY *******" fn /channel add -auto #channel fn
Now /quit and start irssi again and you will automatically conect to irc.freenode.net, join #channel and identify to Nickserv your nick “user”.
To delete the autoconnect configuration use following commands:
/channel remove #channel fn /network remove fn /server remove irc.freenode.net fn
For more IRSSI commands please refer to following link.