The etymological online dictionary says there is doubt about the origin of the word rat and suggests it may be Germanic in origin and transferred from those languages into the Romance languages.
'late Old English ræt "rat," of uncertain origin. Similar words are found in Celtic (Gaelic radan), Romanic (Italian ratto, Spanish rata, French rat) and Germanic (Old Saxon ratta; Dutch rat; German Ratte, dialectal Ratz; Swedish råtta, Danish rotte) languages, but connection is uncertain and origin unknown. In all this it is very much like cat.
Perhaps from Vulgar Latin *rattus, but Weekley thinks this is of Germanic origin, "the animal having come from the East with the race-migrations" and the word passing thence to the Romanic languages. American Heritage and Tucker connect Old English ræt to Latin rodere and thus PIE *red- "to scrape, scratch, gnaw," source of rodent (q.v.). Klein says there is no such connection and suggests a possible cognate in Greek rhine "file, rasp." Weekley connects them with a question mark and Barnhart writes, "the relationship to each other of the Germanic, Romance, and Celtic words for rat is uncertain." OED says "probable" the rat word spread from Germanic to Romanic, but takes no position on ultimate origin. 'http://www.etymonline.com/index.php?all ... hmode=none
The same source has 'rad' being the old English for road. Certainly in Gaelic the word is 'rathad', the 'th' more or less disappears in pronunciation. Perhaps the rathad was the link between the various raths?
And is also Germanic and connected with 'ride' and 'raid'.
'Arad' and 'garri' - The gaelic for 'garden' is 'garradh'. It was originally the wall or dyke around the enclosure but by extension became the enclosure within. There is also a word 'arad' which means strong or brave.