I suppose we should tell the rest of the forum what we are on about.
As with all informal regional words, there are various theories as to the origin of 'marra'.
A former pitman told me miners had a regular buddy at the coal face.
For some reason, that person was known as the miner's 'marrow', and vice-versa.
The term then widely morphed into 'marra', and came to mean any friend - above or below ground.
A quick google suggest the word is of Geordie or West Cumbrian origin, although it's now widely used in Wearside and County Durham - even by some of the young 'uns.
The County Durham and Northumberland miners had their own lingo known as Pitmatic.
https://en.wikipedia.org/wiki/Pitmatic