add unzip to the list of depends for the dmdirc package. pacman needs them so it can unpack the .jar's (which is totally unnecessary)
[Desktop Entry]
Categories=Network;IRCClient;
Comment=DMDirc IRC Client
Encoding=UTF-8
Exec=java -jar /usr/share/java/dmdirc/DMDirc.jar
GenericName=IRC Client
Icon=/usr/share/java/dmdirc/DMDirc.svg
Name=DMDirc
StartupNotify=true
Terminal=false
TerminalOptions=
Type=Application