Arnulph was the fifth son of Roger de Montgomerie, Earl of Shrewsbury and Arundel. He was probably named after his uncle Arnulph de Belàsme. After his father's death, and the assumption of the crown on the death of William Rufus by Henry I., he joined his brothers in resistance to the usurpation, and, as is more fully stated in the account given of Robert de Belàsme, his brother, shared in his ill success, and was banished the kingdom. Arnulph crossed into Ireland in the year 1100 to obtain succors for Duke Robert's cause, and was successful in his mission. He subsequently "sent Gerald, his steward, to Murckhart, or Murtagh O'Brien, King of Munster, desiring his daughter Lafracoth in marriage, which was easily granted." In right of his wife, however, he subsequently aspired to get possession of her father's kingdom. During the invasion of Ireland by Magnus, King of Norway, the Irish, struck with terror, called on the Normans, and Arnulph, who was at Pembroke, collected his auxiliaries and hastened to them. Magnus was repulsed; but the Irish, tasting blood, became more savage and made a sudden attempt to murder the Normans. Their king also carried off his daughter, Arnulph's wife, and resolved to kill Arnulph himself as the reward of his alliance; but he discovered the fraud and made his escape. Of his life he subsequently spent in Normandy, we have but slight account. Ordericus Vitalis states that Arnulph and his brothers Roger, Philip and Everard, "had no share in their father's inheritance, their two elder brothers, Robert and Hugh, having divided between themselves the whole on both sides of the sea." Arnulph had therefore but little if any territory of his own to defend from Henry's anger against the family; and his exertions were all given on his brother's behalf. But "after many thankless enterprises for his brother Robert, he became indignant, and went over to the duke, to whom he ceded the castle of Almenäches which he had taken by surprise." And during Robert's oppressions in SÇes, many of the provincials left him to join Arnulph. This was within two or three years from the time of their leaving England. The next we find of him is in the year 1118, when the Alenáais, desiring to free themselves from the oppressions of Stephen, Count of Mortain, their new master, they, in concert with Arnulph, secretly called to their succor the Count of Anjou, who finally conquered the city and castle. The count held them until the year following, when by treaty remitting them to Henry I. for that purpose, the latter reinstated Arnulph's nephew William in their possession. About a year later than this, we find he had returned to Ireland, was reconciled to his father-in-law to outward appearances at least, and was reunited to his wife; but on the morrow of his new nuptials fell asleep after a banquet, from which he never awoke. The castle of Pembroke had been built by him, probably before his father's death; and in the year 1095 it was strong enough to resist all the attempts of the Welsh. The first one built at Pembroke is described as "a poor, slight castle of stakes and turf, which afterwards, on his return to England, he left to the care of a discreet man, Giraldus de Windesor, his constable and chief officer, with a small garrison, who were presently besieged by the whole force of South Wales. But Giraldus made such a vigorous defence, rather by courage than strength, that they were obliged to retire without success." "That this powerful and proud Norman found a fortress there of the character thus ascribed to it is probable, but it is not possible that he should have long suffered the capital of his newly-acquired possessions to continue so slenderly protected. . . . . The superior grandeur that characterizes the ruins of the castle and town of Pembroke, with its various magnificent appendages, enable us to form an estimate of the importance Arnulph de Montgomerie attached to his new conquest."
Source: "Genealogical History and Pedigree of the family of Montgomery" Thomas Harrison Montgomery Call Number: R929.2 M788