Money in general. To polish the king’s iron with one’s eyebrows; to look out of grated or prison windows, or, as the Irishman expresses them, the iron glass windows. Iron doublet; a prison. See Stone Doublet.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Irish Toyles * Ironmonger’s ShopNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes