Origin of the war-cry "Eulalia" in Brian Jacques's Redwall series?

Question

In Brian Jacques's Redwall series, "Eulalia" is the traditional war cry of the badgers and hares of Salamandastron. It's also the title of one of the books in the series, but it's used throughout all books as a battle cry.

What is the origin of this war cry? I'm interested in both "in-universe" answers (does it have any meaning in the world of the Redwall books, or how did it come to be the battle cry of Salamandastron) and "out of universe" answers (why did Brian Jacques choose this word to use as a war cry in his books, does it relate to either of the Saints Eulalia or any other real-world usage of the word).

Answer 1

According to Brian Jacques, as related in a live-action segment for the TV series:

That was one of the Norse war cries, the Vikings, the sea-wolves, the Norsemen, and the Celts used to use it when they went into battle.

This is also stated in the Redwall FAQ:

Eulalia is a Celtic/Norse battle cry which means Victory!!

This explanation has been noted to not match historical records, but it is the official answer from the author.

Answer 2

The historian T. L. Kington Oliphant claimed that “Eulalia” was (part of) the war-cry of Barcelona during the War of the Spanish Succession and afterwards. (Saint Eulalia is the patron saint of Barcelona, in whose cathedral she is buried.)

I have no evidence that this was (by whatever chain of whispers) ultimately the source for Brian Jacques: I merely offer it as a curiosity.

In 1705 Valencia declared for Charles the Austrian,¹ and was soon followed by Catalonia, a province that had good reason to hate anything coming from France. Peterborough² took the fortress³ that overawed Barcelona, a feat that even Marlborough⁴ or Eugene⁵ might have envied. The noble old city for many years continued to utter her war-cry, “St. Eulalia and Charles the Third.” The Catalans had none of that feeling which in Castile caused medals to be struck inscribed with “Charles III, by the grace of the Heretics, Catholic King.” Last of all, Aragon went over to the side of Charles; all the monks and friars took up arms, and Peterborough, after reviewing a number of these at Valencia, declared that he had beheld the Church militant.

_{Thomas Laurance Kington Oliphant (1902). Rome and Reform, volume I, p. 300. London: Macmillan.}

_{¹ Later Charles VI, Holy Roman Emperor ² Charles Mordaunt, 3rd Earl of Peterborough ³ In the second siege of Barcelona ⁴ John Churchill, 1st Duke of Marlborough ⁵ Prince Eugene of Savoy}

Unfortunately Kington Oliphant is not very diligent about referencing his sources. For this passage he writes, “Most of what follows is contained [in Labat, i.] 227–385”, which I take to be Jean-Baptiste Labat (1730), Voyages en Espagne et en Italie, volume I, but there is no mention there of the war-cry, so I do not know where he got this detail from.