Una Due Tfc
Your squawking 77 overrides all those filters and sets off audio alarms in both Shannon and Dublin. Everyone in every sector sees your mode c rolling like a slot machine and starts moving traffic in their sectors out of your way.
Over the years we have had the debate of whether we should squawk 7700 when making an emergency descent. This has varied from yes we should squawk 7700 to alert ATC or no we shouldn't as ATC will lose all screen data on us, aircraft type, callsign etc. Generally we understand the preference to be not to squawk 7700 unless ATC request it or if we are unable to contact them. Consequently to squawk 7700 in emergency descent was removed from the checklist a while ago.