/usr/include/irssi/src/irc/ is in irssi-dev 0.8.15-5ubuntu3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/irssi/src/irc/core/
/usr/include/irssi/src/irc/core/bans.h
/usr/include/irssi/src/irc/core/channel-rejoin.h
/usr/include/irssi/src/irc/core/ctcp.h
/usr/include/irssi/src/irc/core/irc-channels.h
/usr/include/irssi/src/irc/core/irc-chatnets.h
/usr/include/irssi/src/irc/core/irc-commands.h
/usr/include/irssi/src/irc/core/irc-masks.h
/usr/include/irssi/src/irc/core/irc-nicklist.h
/usr/include/irssi/src/irc/core/irc-queries.h
/usr/include/irssi/src/irc/core/irc-servers-setup.h
/usr/include/irssi/src/irc/core/irc-servers.h
/usr/include/irssi/src/irc/core/irc.h
/usr/include/irssi/src/irc/core/mode-lists.h
/usr/include/irssi/src/irc/core/modes.h
/usr/include/irssi/src/irc/core/module.h
/usr/include/irssi/src/irc/core/netsplit.h
/usr/include/irssi/src/irc/core/servers-idle.h
/usr/include/irssi/src/irc/core/servers-redirect.h
/usr/include/irssi/src/irc/dcc/
/usr/include/irssi/src/irc/dcc/dcc-chat.h
/usr/include/irssi/src/irc/dcc/dcc-file-rec.h
/usr/include/irssi/src/irc/dcc/dcc-file.h
/usr/include/irssi/src/irc/dcc/dcc-get.h
/usr/include/irssi/src/irc/dcc/dcc-queue.h
/usr/include/irssi/src/irc/dcc/dcc-rec.h
/usr/include/irssi/src/irc/dcc/dcc-send.h
/usr/include/irssi/src/irc/dcc/dcc-server.h
/usr/include/irssi/src/irc/dcc/dcc.h
/usr/include/irssi/src/irc/dcc/module.h
/usr/include/irssi/src/irc/flood/
/usr/include/irssi/src/irc/flood/module.h
/usr/include/irssi/src/irc/notifylist/
/usr/include/irssi/src/irc/notifylist/module.h
/usr/include/irssi/src/irc/notifylist/notify-setup.h
/usr/include/irssi/src/irc/notifylist/notifylist.h