The Attacotti, Scotti, Picts, and Saxons mentioned
by the historian Ammianus are clearly mentioned as different
peoples. All of the Irish are lumped together as Scots. The Picts
are mentioned as one group. The only Germans mentioned are Saxons,
but it was common practice in Roman Britain for any encountered
sea-borne Germans to be labelled Saxons.
This all implies that the Attacotti were members
of another ethnic group. But if so, where did they come from? There
are no others in the region. The only peoples in the British Isles
at this time are Germans (Saxons, Frisians, etc), Picts, Scots,
and Britons. So could the Attacotti be Britons?
Saint Jerome's Against Jovinianus mentions
numerous tribes and peoples by name and differentiates between
them strongly. St Jerome says, 'I myself, a youth on a visit to
Gaul, heard that the Atticoti, a British tribe...' ('cum ipse
adolescentulus in Gallia viderim Atticotos, gentem Brittanicam').
This was in the fourth century AD, the middle 300s,
and in that period the only Britons outside the empire (though
not outside its influence) were in the mostly abandoned region
north of Hadrian's wall, south of the Antonine Wall (and possibly
including the 'lost' province of Valentia).  The tribes between
the two walls at that time seemed to have formed client kingdoms,
and they knew they needed to behave themselves or a Roman legion
could appear and take their forts.
Only one tribe or kingdom of Britons had a fort
which could not be easily reduced by the Roman engineers. This was
Alt Clut, the 'rock of the Britons'. It was on top of a high rock
with many vertical sides, protected by water on three sides. And
it was to the north of the Antonine Wall.
If one removes the two 'l' from Alt Clut, one gets
'At Cut', easily simplified or mispronounced as 'Atcut', which would
then be Latinised as 'Atticoti' when written in Latin. The Britons of
Alt Clut had a tradition of calling themselves the last free Britons.
As such they would resent the Roman occupation and would be willing
to attack Roman Britain to their south.
An example of this behaviour comes a century later
in the epistle to Coroticus which was written by a Briton known today
as Saint Patrick (see external link, right, for the full epistle).
In that letter Patrick complains bitterly to Coroticus, the king of
Alt Clut, because Coroticus' soldiers had attacked Patrick's Christian
converts in Ireland, murdering some and taking captive others who were
then sold to the Picts. Patrick was a Roman Briton, and Christianity
was in that period regarded as a Roman religion to be shunned by the
pagans of the north. The original 'barbarian conspiracy' of Scots,
Picts, Saxons, and Attacotti would have been an alliance of pagans
who were opposed to Roman Christianity and all it stood for (the
empire in particular).
They would appear to be the perfect candidates for
the Attacotti - Alt Clut, otherwise known at a later date as
Dumbarton, the Rock of the Britons, today is still a formbidable
obstacle, although the defences of its British occupiers were
finally breached in 870-871