Hi Gus,
all I could find was this:
"
The Metrical DindshenchasMag Muirthemne, whence the name? Not hard to say. The sea covered it thirty years after the Flood, and hence it is called Muirthemne, that is, ‘darkness of the sea’, or ‘it is under the sea's roof’. Or there was a magic sea over it, and an octopus therein, having a property of suction. It would suck in a man in armour till he lay at the bottom of its treasure-bag. The Dagda came with his 'mace of wrath' in his hand, and plunged it down upon the octopus, and chanted these words: ‘Turn thy hollow head! Turn thy ravening body! Turn thy resorbent forehead! Avaunt! Begone!’ Then the magic sea retired with the octopus; and hence, may be, the place was called Mag Muirthemne."
Then looking I found this on Wikipedia about Nemed from the Book of Invasions...
http://en.wikipedia.org/wiki/Nemed:
"He cleared twelve plains: Mag Cera, Mag Eba, Mag Cuile Tolaid, and Mag Luirg in Connacht: Mag Seired in Tethba; Mag Tochair in Tir Eogain; Mag Selmne in Araide; Mag Macha in Airgialla;
Mag Muirthemne in Brega; Mag Bernsa in Leinster; Leccmag and Mag Moda in Munster."
These two links provide maps of Brega but don't mention Mag Muirthemne
http://www.rootsweb.ancestry.com/~irlkik/ihm/mide.htmhttp://www.rootsweb.ancestry.com/~irlkik/ihm/suineill.htmA google book search suggested 'an area around Dundalk, Co Louth' and also found this
http://books.google.co.uk/books?id=MPZiWhhAmXAC&pg=PA47&lpg=PA47&dq=mag+muirthemne+brega&source=bl&ots=UnPN55ZbSV&sig=3KwN5Z7V4JERXYSoj7mREbKKsw4&hl=en&sa=X&ei=0oqWT8yzEYWG8gOahPWRCg&ved=0CDcQ6AEwAw#v=onepage&q=mag%20muirthemne%20brega&f=false
Most dear is fire to the sons of men,
most sweet the sight of the sun;
good is health if one can but keep it,
and to live a life without shame. (Havamal 68)
http://gewessiman.blogspot.co.uk