Diplomacy in potential hostile regions

[Nathan Ross]

perhaps we go with a small, private unit to protect the merchant in case things go wrong.

Hmm, this is getting interesting! I'd agree with Mark that turning up with a regular military detachment would seem a bit overly hostile. For a 'private unit' (considering this is late Roman Britain and things might be getting a little rough around the edges...) the following could be possible, I'd say:

1. Unofficial soldiers. With the difficulties in paying and provisioning soldiers in the provinces, it might have possible for an enterprising merchant to cut a deal with the commander of a frontier or coastal garrison (or even, if the merchant was very wealthy or well-connected, with the governor or military commander of the province) to 'lend' him a few men for back-up, in return for a cut of the proceeds... The soldiers would perhaps have kept their weapons and military equipment out of sight unless it was needed... Could make for some unusual dynamics in the command hierarchy!

2. Armed slaves. Any merchant would have a retinue of slaves to act as porters, bearers, rowers (see below) etc, and in the late empire we do hear of landowners and others arming their slaves as (in some cases) private armies. No reason why a trader to Ireland would not have done this.

3. Hired barbarians. Alternatively, he may have hired some 'military contractors' from abroad... We don't really know how and when various barbarian groups arrived in Britain, but warriors from mainland Europe (Franks, Frisii, perhaps Saxons) were used by the usurper Allectus in the AD290s, and quite possibly other groups may have been around later, either as irregular troops, ex-soldiers or just mercenaries. Several villa sites in western Britain have 'Germanic' brooches and military gear from the 4th-5th centuries, so local landowners may have been hiring their own barbarian bodyguards as central control slipped away...

4. Rival Irish. Tacitus's Agricola mentions that the Romans knew all about 1st century Ireland from rebels and exiles who had crossed to Britain, and there's no reason why this shouldn't have happened later too. A trader with links to Ireland may have decided to take some Irish migrants back with him, both as interpreters and as protection. Of course, that might have led to problems with the 'new' king as his people, if they had issues with the migrants... but perhaps that just makes things more interesting?

Depending on which option the merchant went for, and how many men he had with him of course, there may have been difficulties with the official Roman authorities and/or army as he tried to leave and reenter the province: he wouldn't want to be mistaken for a raiding party or a pirate expedition, on whichever side of the Irish Sea!

Galley slaves is a late Medieval/Renaissance practice... in Antiquity, rowers were freemen

Much as it galls me to say so (as I've spent years arguing against the 'galley slaves' cliche!) in the case of private merchant ships (rather than the navy) the oarsmen possibly would have been slaves... although Synesius of Cyrene claims his ship's crew were partly "a collection of peasants who even as late as last year had never gripped an oar"...!