Upright Citizens

Gathering up the info, if only for my own future reference!

This achievement can only be done while the quest Not Too Sober Citizens Brigade is active during an Assault on Tiragarde Sound.
It is not always active during an assault and there is no pattern as to when it will be - which quests are active seems to be completely random.
Only one of the 3 named NPCs will be spawned during any one Assault, so it will take at least 3 Tiragarde Assaults to complete this.

Initially the evidence suggested that in any given month the same NPC would spawn if the quest was up; first reports of Leeroy being spawned were all in December 2018, Flynn throughout January and February 2019 with Russel in March and April the same year. It looked like it may have even been a bi-monthly rotation.

However, May 2019 saw both Leeroy and Russel spawn in different assaults, pointing to the rotation being completely random or possibly the introduction of a new rotation - the chance of the first 5 months to produce the results they did randomly are pretty astronomical. With less downtime between the 3 NPCs though, it now looks like we should be able to complete the achievement much quicker regardless.

When spawned you can find the NPCs at the following locations:
[npc=145107] in Mariner's Row at 66.8, 64.1
[npc=146295] in Mariner's Row at 72.3, 61.6
[npc=145101] inside The Curious Octopus at 72.7, 69.2

TomTom Waypoints:
/way Boralus 66.8, 64.1 Leeroy Jenkins
/way Boralus 72.3, 61.6 Flynn Fairwind
/way Boralus 72.7, 69.2 Russel the Bard

I found Leeroy jenkins at 67,64 in Mariners row in Boralus during an assault. Could not find the others. Maybe only one character spawns per event?

Useful macro to locate them and place a marker on their head:

/way 72.3 61.6 Flynn
/way 66.8 64 Leeroy
/way 72.4 68.9 Russel
/tar Russel the Bard
/tar Flynn Fairwind
/tar Leeroy Jenkins
/script SetRaidTarget("target",8)

Edit: as others have posted, only one of the three may be up in one Horde invasion of Tiragarde Sound. You may have to wait several weeks to get all three of them.

Update: 6 May 2019, Leeroy finally back up on EU realms. Added co-ords for each one.
[Forbidden Sea Shanty of the Lively Men] is on the fireplace next to where Russel appears.