Maybe I am thinking Doric rather than Gaelic... in my opinion :) I'll translate a bit using Doric as a guide :)

Looks like the first line is an address:
Main = Home farm/estate, allarbr... a place?... The next word I presume, is an area in Ayrshire, then Ayrshire.

It's hard to read, but the second line, "muir Irvine", if it is Doric, would mean moor Irvine. As in, the moors around Irvine.

Irin Roffno mentioned is a name,(Irin is a version of Irene)... But I don't think that is what it means here. Roffno I suggest is a name, but Irin or Imin... It comes up like we might use mr or miss, don't you think?... I don't know what that means... Johnny W again a name... Ann seems to be someone they all know, because she does not have a surname... If I was you, I might link the letter to the surname Roffno.

The "Depot" is mentioned a lot, I presume that means the same as it does now...

Line 5, "bride" means to "live"... Line 5 and 14 and the last but one lines, "gang" means "go"... Lines 8, 9, 10 and 11 have "fin" somewhere near the end, which means "when", in Doric.

Its such a strange letter! I looked at the first page only, but phrases like "pin down", "down pin", and "down", seemed to come up again and again! Could someone have been picking up the local dialect too? It doesn't sound a Scottish phrase...

I would be pretty sure its Doric, but you can't translate it to English directly, there are words in Doric that don't exist in English. It sounds like a very interesting letter, but it would take ages to translate as it's so hard to read. But you can get books to help...