This is the description of what Mahon MacMahon is wearing, but in today's Irish would the word gúna be understood as a 'gown' as well as meaning a girl's dress or would another word be more appropriate ?

Here's the sentence and my attempt at translating it. As usual I'm not quite sure about the word order and I would be very grateful for any corrections.

But he walked along the hall, by the side of Mahon, as if nothing was the matter, although his (Mahon's) great iron dress clanked fearfully at every step, sounding louder than Robin's own sledge battering on his anvil.

Ach shiúil sé feadh an halla le taobh Mathúna, mar ba chuma leis, cé gur dearna gliogarnach gurbh uafásach a ghúna mhór iarainn ar gach uile choiscéim agus bhí ní ba chlingi ná trup clagarnach an oird ceárta Róibín féin ar inneoin.

In Irish, would it be necessary to underline that this 'iron dress' is worn by Mahon and not by Robin ?

Culaith can mean suit or dress, I think that could work.

Culaith iarann maybe?

Go raibh maith agat.

Just another question - would iarann not be in the genitive .i. a chulaith iarainn ?