The castle overlooks the oldest street in Irvine, which was once the main route between the town and the old harbour at Seagatefoot, which by 1606, was useless and abandoned due to silting.

[5] Irvine was created a Royal Burgh by King Robert II in 1372[6] and this castle is the last of the town's old civil and ecclesiastical buildings to survive, the Carmelite friary,[7] tollbooth, chapel, bridge, and port having vanished without trace.

[8] The original, probably-wooden castle tower or motte, was therefore built some time before 1184, being rebuilt in stone in the 1360s and then remodelled and expanded by Hugh the 3rd Earl of Eglinton, around 1565.

In the twelfth century the castle of Irvine lay within the lordship of Cunninghame, which had been granted by David I to Hugh de Morville, Lord High Constable of Scotland.

Elizabeth de Eglintoun, as sole heiress, married John Montgomerie of Eaglesham, to whom the estates and possessions passed, including the barony of Ardrossan.

[16] The castle is entered through a vaulted pend running through the north section of the main block, which has a fine ornamental arched doorway, opening into the courtyard and having similarities with the southern entrance passageway at Linlithgow Palace.

[17] The south half of the long main block and one round tower now are reduced to the basement and foundations, but the remainder is fairly entire as far as the wallhead.

The ground floor is fairly complete with vaulted accommodation, with two guard rooms, each having a single door; a typical defensive precaution of the time.

[17] The ornate entrance off Seagate has attracted some debate as to its origin, being visually more ecclesiastical than baronial in appearance, variously described as 'Saxon', 'Norman', or 'Scottish Gothic' in style.

McGibbon and Ross were of the opinion that the doorway is quite typical of sixteenth and seventeenth century Scottish Renaissance Gothic work as found in several collegiate and parish churches and not necessarily Norman in origin.

When inspected in 1883 the vaults, kitchen, and other apartments, were found full of rubble, soil, and rubbish of all kinds, much of which was the refuse chiefly of adjoining properties.

[31] In the 1880s at the Marymass Fair, the carters, freemasons, and others on their return from the parade on the racecourse, came back via the Seagate, stopped and arranged themselves in front of the old Castle to sing " Auld Lang Syne.

In 1297 Edward I had sent a punitive expedition under Sir Henry Percy to Irvine to quash an armed uprising against his dethronement of John Balliol.

The Earl of Carrick, Robert Bruce, Bishop Wishart and others led the Scottish army, however, after much argument they decided to submit without a fight.

An Irish serving-maid was accused of stealing some silverware, however, she declared herself to be innocent and in a fit of anger offered to raise the Devil, who would reveal the identity of the real thief.

She was not taken seriously, however, she set about this task, entering the cellar and drawing a circle around herself and performing a complex ritual with a Bible, riddle, and a black cock's feathers.

[36][37] Whilst the ritual was being performed the general and his wife had been struck with an inexplicable sense terror and the dogs of the town had been barking in fear.

[38] Seagate was found to be in danger of partial collapse in 2010 and in 2011 North Ayrshire Council carried out repair works to consolidate the structure of the castle.