/usr/include/irssi/src/core/ is in irssi-dev 1.0.5-1ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/irssi/src/core/args.h
/usr/include/irssi/src/core/channel-rec.h
/usr/include/irssi/src/core/channel-setup-rec.h
/usr/include/irssi/src/core/channels-setup.h
/usr/include/irssi/src/core/channels.h
/usr/include/irssi/src/core/chat-protocols.h
/usr/include/irssi/src/core/chatnet-rec.h
/usr/include/irssi/src/core/chatnets.h
/usr/include/irssi/src/core/commands.h
/usr/include/irssi/src/core/core.h
/usr/include/irssi/src/core/expandos.h
/usr/include/irssi/src/core/ignore.h
/usr/include/irssi/src/core/levels.h
/usr/include/irssi/src/core/line-split.h
/usr/include/irssi/src/core/log.h
/usr/include/irssi/src/core/masks.h
/usr/include/irssi/src/core/misc.h
/usr/include/irssi/src/core/module.h
/usr/include/irssi/src/core/modules-load.h
/usr/include/irssi/src/core/modules.h
/usr/include/irssi/src/core/net-disconnect.h
/usr/include/irssi/src/core/net-nonblock.h
/usr/include/irssi/src/core/net-sendbuffer.h
/usr/include/irssi/src/core/network.h
/usr/include/irssi/src/core/nick-rec.h
/usr/include/irssi/src/core/nicklist.h
/usr/include/irssi/src/core/nickmatch-cache.h
/usr/include/irssi/src/core/pidwait.h
/usr/include/irssi/src/core/queries.h
/usr/include/irssi/src/core/query-rec.h
/usr/include/irssi/src/core/rawlog.h
/usr/include/irssi/src/core/recode.h
/usr/include/irssi/src/core/server-connect-rec.h
/usr/include/irssi/src/core/server-rec.h
/usr/include/irssi/src/core/server-setup-rec.h
/usr/include/irssi/src/core/servers-reconnect.h
/usr/include/irssi/src/core/servers-setup.h
/usr/include/irssi/src/core/servers.h
/usr/include/irssi/src/core/session.h
/usr/include/irssi/src/core/settings.h
/usr/include/irssi/src/core/signals.h
/usr/include/irssi/src/core/special-vars.h
/usr/include/irssi/src/core/tls.h
/usr/include/irssi/src/core/utf8.h
/usr/include/irssi/src/core/window-item-def.h
/usr/include/irssi/src/core/window-item-rec.h
/usr/include/irssi/src/core/write-buffer.h