/usr/include/irssi/src/irc/core/ is in irssi-dev 1.0.5-1ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/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-cap.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/sasl.h
/usr/include/irssi/src/irc/core/servers-idle.h
/usr/include/irssi/src/irc/core/servers-redirect.h