The castle is situated in the portion of Leixlip which extends into North Salt Barony in the County Kildare. The fortress occupies a commanding position at the juncture of the Rivers Rye-water and Liffey, above the famous Salmon Leap from which the designation Leixlip is derived, being a Danish name from the old Norse word “Lax-hlaup,” i.e., Salmon Leap. From the word “Saltus,” a leap, the baronies of Salt have also taken their name.

The castle is generally supposed to have been erected by the De Hereford family towards the close of the twelfth century. The present building consists of two blocks at right angles, facing east and south. The east wing probably incorporates part of the twelfth-century keep, and with the north-east circular tower represents the oldest portion of the structure, although it has been pierced by modern windows.

In this part a room is still shown in which tradition states that King John slept during his stay in Ireland.

The square south-east tower is not so old, and its erection is generally ascribed to the Geraldines.

The lands of Leixlip were granted to Adam de Hereford by Strongbow shortly after the Normans arrived in 1170. At the close of the thirteenth century the castle and lands had passed to the Pypards. In 1302 Ralph Pypard surrendered all his castles, &c., to the Crown, and in consequence Richard de Bakeputz, who was constable of Leixlip, was ordered to deliver it up to the King.

Leixlip Castle was included in the list of those fortresses that were only to have Englishmen as constables by the statute passed in 1494.

Henry VII. granted the castle and lands to Gerald, 8th Earl of Kildare, upon his marriage with Dame Elizabeth Saint John, between the years 1485 and 1509, and they remained in the possession of the FitzGeralds until the rebellion of “The Silken Thomas” in 1534, in which the owner, Sir James “Meirgach” (i.e., the winkled) FitzGerald was concerned. In 1536 an Act was passed by which the Crown became possessed of the castle and manor.

Two years after Mathew King, of Dublin, surrendered the castle, which appears to have been granted to him for twenty-one years. In 1568 William Vernon, gentleman, was leased the manor of Leixlip, containing castles, &c., by the Crown. Nine years later Sir Nicholas Whyte, Master of the Rolls, came into possession. He was a son of James Whyte, of the County Waterford.

In 1570 he was granted the manor of Leixlip, two castles, a water-mill, a salmon-weir, two fishing-places called the Salmon Leap, on the River Anna Liffey, Priortown Meade, and other demesne lands. Two years later he was made Master of the Rolls.

Sir Nicholas Whyte was succeeded by his son Andrew, whose son was again Sir Nicholas Whyte, Knt.

This Sir Nicholas held the manor of Leixlip upon the breaking out of hostilities in 1641. In company with Lord Dunsany, Patrick Barnwall, Sir Andrew Aylmer, and other chief men of the Pale, he surrendered himself to the Lords Justices Parsons and Borlace. This was done in obedience to the King’s proclamation to show that they had no part in the rebellion, but they were imprisoned in Dublin Castle and most inhumanly treated.

In the diary of Captain William Tucker he records going from Dublin to Naas in 1641 with the Marquis of Ormond, and sleeping a night in Leixlip Castle. He mentions that the owner, Sir Nicholas Whyte, was at the time a prisoner in Dublin.

In 1646 General Preston established his chief quarters in the fortress, and in November of that year the Confederate Catholics halted in their march on Dublin between Leixlip and Lucan. They were commanded by Generals Preston and Owen Roe O’Neill. The King’s secretary and minister, Digby, was at Leixlip with Preston.

Plots and counter plots among the Confederate commanders made the once formidable army of no avail. Owen Roe, fearing some treachery, threw a wooden bridge across the Liffey, as a flood had destroyed the permanent one, and withdrew his forces into Meath.

Sir Nicholas Whyte recovered his lands of Leixlip by a Decree of Innocence. He died in 1654, and was buried at Leixlip.

Various historians have confused the fortress popularly called Leixlip Castle with a stronghold of less note designated the “Black Castle” of Leixlip, situated at the eastern end of the town. Although still known as the “Black Castle” this building has been so modernised that its original fortified structure is not noticeable.

That some discrepancies as to ownership existed in the written history of Leixlip Castle was first noted in 1901, but it was not until the following year that Lord Walter FitzGerald, in a note in the Journal of the Kildare ArchÆological Society, gave an extract from “The Civil Survey” of James Peisley and Henry Makepeace of 1654, in which the “Black Castle” of Leixlip is mentioned as belonging to the Earl of Kildare and “one ruined castle” to Sir Nycholas White, thus establishing the fact that there were two distinct castles at Leixlip owned by different persons.

The “Black Castle” is therefore no doubt the fortress alluded to in an inquisition held in September, 1612, which states that Gerald FitzGerald, son of Gerald, late Earl of Kildare, and uncle of Gerald, late Earl of Kildare, was seized of one castle, three messuages, one ruined water-mill, and forty acres of arable land at Leixlip. And again in 1621 the inquisition taken upon the death of Gerald, 15th Earl of Kildare, includes the Castle of Leixlip, &c. While the rental of the Earl of Kildare in 1657 mentions the black castle of Leixlip with sixty acres of land valued at £15 a year.

Leixlip Castle was purchased by the Right Hon. William Conolly, Speaker of the Irish House of Commons, at the beginning of the eighteenth century. He subsequently built the mansion of Castletown at Celbridge, but his nephew and heir occupied Leixlip Castle during the famine years of 1740 and 1741.

After this period the castle has been inhabited by many distinguished tenants.

It was a favourite residence of Primate Stone, and during Lord Townshend’s period of office he usually passed the summer there.

Many stories are told of this Viceroy’s fancy for mixing incognito with “all sorts and conditions of men.”

One day Lord Townshend met a journeyman cutler named Edward Bentley in the demesne of Leixlip Castle and began to talk to him. Bentley was loud in his praises of the Lord Lieutenant’s kindness in allowing the public into the grounds of his residence, but he was equally vehement in denouncing the political views he held.

Mistaking the proprietor for one of the retainers, he offered him half a crown upon leaving, and when it was refused the cutler commented on the difference between his action and that of the gate-keeper who had demanded that amount.

Lord Townshend then took him to the castle and provided him with a cold repast, but as he was escorting his departing and grateful visitor through the hall the unfortunate gate-keeper came in.

The Lord Lieutenant asked him why he had dared to disobey orders and receive money from visitors. Whereupon the man fell upon his knees and asked pardon. Bentley, at last realising who had been his entertainer, immediately followed suit. Lord Townshend sent for his sword, and the cutler was quite certain that his last hour had come. The Lord Lieutenant flourished the weapon over his head and brought it down smartly on the terror-stricken man’s shoulder, saying, “Rise, Sir Edward Bentley.” The new-made knight was appointed cutler to His Excellency, and lived long to enjoy his honour.

Viscount Townshend’s wife died at Leixlip Castle.

The Hon. George Cavendish remodelled the building and brought it up to modern requirements during his tenancy before 1837.

John Michael, Baron de Robuck, subsequently lived there, and was drowned in the Liffey in 1856 during a flood.

In 1878, Captain the Honourable Cornwallis Maude, son of the Earl of Montalt, took the castle after his marriage. He was killed at Majuba Hill.

The present occupier is William Mooney, Esq., J.P.

This fortress is one of the oldest inhabited houses in Ireland. It has been said that the novelist Maturin founded one of his weird plots on a legend relating to Leixlip Castle, but the statement requires verification.

An underground passage runs from the castle, beneath the Byewater, to St. Mary’s Church, where it terminates in a vault under the building, the end being now walled up.