Green’s Dictionary of Slang

an Irish Catholic who rejected nationalism, preferring to curry favour with and ape the lifestyle of the ruling British.

1848		Freemans Jrnl (Dublin) 6 Dec. 2/4: That is too respectable for even Castle Catholics—it must be reserved for the true blues.
1852		Nation (Dublin) 17 Jan. 10/4: There were Catholics on this list, of course—Castle Catholics, employees in public offices, the brothers, and brothers-in-law of official people, the ‘sons of Baronets’ made by the Whigs [...] in short, your sworn servants.
1862		Irish Examiner (Cork) 9 Aug. 3/3: The expressions [...] must ‘have been taken exception to’ by the Castle Catholics, or the ‘gentilities’ to use his Grace’s own epithet.
1872		Nation (Dublin) 23 Nov. 12/3: It is almost certain, however, that Palles has no chance, unless the Presbyterian Whigs prefer a Castle Catholic to a Home Rule Protestant.
1882		Nation (Dublin) 16 Sept. 10/1: The soul-less creature known at the present day as the Castle Catholic was not unknown in 1641, and that he was regarded then by the patriotic party much as he is regarded now by the mass of the people.
1892		Tuam Herald 9 Jan. 2/3: The Government will be obliged to appoint some shady Castle Catholic, or perhaps some Protestant.
1908		L. Paul-Dubois Contemp. Ireland 465: There is, indeed, a small minority of Catholics who cling on to the skirts of the Ascendancy party, those ‘Castle Catholics’ of whom Daniel O'Connell used to say ‘The Lord forgive me for having emancipated such fellows as those’.
1918		F. Hackett Ireland 116: A few of them [...] would have become fine parish priests. But, whatever they did, short of becoming ‘Castle Catholics,’ they would still be ‘natives’.
1938		S. O’Faolain King of the Beggars n.p.: A class that became known as ‘renegade Catholics’ or, in mockery of their affected half-English accents, ‘Cawstle Catholics’ [BS].
1958		P. Jullian Snob Spotters Guide 97: There was a section of the catholics who preferred the Viceroy's entertainment to licking their wounds outside the Castle — these were known as ‘Castle Catholics’.
1968		D. Neligan Spy in the Castle 45: The British had many supporters here: the landlords, big business, the banks, the Freemasons, non-Catholics, Castle Catholics, nearly all the rich and educated people.
1979		C.S. Andrews Dublin Made Me 10: The Castle Catholics [...] lived cheek by jowel with the Protestants on Mountjoy Square, Fitzwilliam Square and Merrion Square .
2013		A. Jackson Buildings of Empire 27: The 'Castle Catholics'—a pejorative term applied by nationalists to those Catholics admitted to viceregal circles [...] —were a largely petit-bourgeois element.

an informer.

1914

Joyce ‘Ivy Day in the Committee Room’ Dubliners (1956) 122: ‘O, but I know for a fact,’ said Mr Henchy. ‘They’re Castle hacks.’.