Diplomacy in potential hostile regions

[Nathan Ross]

The time frame is late 4th century... The setting would be the coast of Ireland

Relations between Roman Britain and Ireland are pretty vague at the best of times. There was a new coastal fort built at Lancaster, probably in the late 3rd or early 4th century and perhaps garrisoned by the numerus barcariorum whose praepositus (commander) Sabinus dedicated an altar to Mars at nearby Halton. These would have been naval troops ('boatmen'), presumably intended to intercept seaborne raiders from Ireland - Scotti - or Picts from further north.

So there must have been some traffic across the Irish Sea, and it probably wasn't entirely hostile all the time... Most likely traders operating in those waters would have known about the local chiefs or kings on the Irish side, as you suggest, and the Romans may have had treaties with them or paid some of them subsidies (or bribes) to keep the peace or provide slaves or perhaps even military recruits. This would surely have involved some sort of diplomatic visits across the sea, although who was doing it and when is a mystery.

There probably wasn't much of a link between traders and soldiers though - unless the soldiers were doing a bit of speculation on the side!

How would they indicate to the locals that they have come in peace, despite the fact they are armed?

One suggestion I've come across a few times is the use of a leafy green branch as a sign of peace - I actually have no idea where this comes from, or whether it has any basis in fact or is entirely apocryphal, but it seems a good enough guess! Of course, anyone coming across the sea would have to bring it with them...