The Menapii were a Belgic tribe of north-eastern Gaul in the 1st century BC, dwelling around the Rhineestuary, and extending inland towards the Ardennes. Their chief town was Cassel. The Menapii are also attested as living in north-eastern Ireland.
The Menapii bordered on the territories of the Eburones, and were protected by one continued extent of morasses and woods; and they alone out of Gaul had never sent embassadors to Caesar on the subject of peace.
Ho thought that these auxiliaries ought to be detached from him before he provoked him to war; lest he, despairing of safety, should either proceed to conceal himself in the territories of the Menapii, or should be driven to coalesce with the Germans beyond the Rhine.
Caesar, after he came from the territories of the Menapii into those of the Treviri, resolved for two reasons to cross the Rhine; one of which was, because they had sent assistance to the Treviri against him; the other, that Ambiorix might not have a retreat among them.
The Menapii, a Gaulish maritime tribe inhabiting the dense forests of the Rhineestuary on the North Sea Coast, were first mentioned by Julius Caesar in 57 B.C. During the Gallic War he singled them out as the only tribe never to surrender to his legions.
The Menapii are the only known Celtic tribe specifically named on Ptolemy's 150 A.D. map of Ireland, where they located their first trading colony -- Menapia -- on the Leinster coast circa 216.D. They later settled around Lough Erne, becoming known as the Fir Manach, and giving their name to Fermanagh and Monaghan.
As I mentioned, the Menapii had to resist the encroachments of marauding Germanic tribes and then Caesar's Roman Legions, but through their use of guerilla tactics, he was never able to totally conquer them.