Agnes Mary Frances Castle (née Sweetman; c. 1860/1861 – 1922) was a Victorian era Irish author who worked with both her sisters and husband.

Agnes Mary Frances Sweetman was born in County Dublin to Michael James Sweetman (1829-1864), of Lamberton Park, Queen's County, JP, High Sheriff of Queen's County in 1852, and (Mary) Margaret, only child and heir of Michael Powell, of Fitzwilliam Square, Dublin.

The Sweetman family were landed gentry of Longtown, County Kildare, and per family tradition were "long settled in Dublin" and "previously resident near Callan and Newtown, County Kilkenny", tracing their line back to the mid-1500s.

After her father's death, when she was a small child,[5] the remaining family moved to Brussels in 1873 and she spent her summers in Switzerland.

[10] Her daughter, Marie Louise Egerton Castle (1885-1969), made a "considerable reputation" as author of a "History of Italian Literature" and reviser of Cary's translation of Dante's Divine Comedy; on 25 October 1922, at the Brompton Oratory, she married Count Antoine de Meeûs, of Brussels.