It probably was, actually.

At the same time you have Alfonso of León crowining himself Imperator Totius Hispaniae or Imperator Hispaniarum (Emperor of All the Spains, in plural) - But the idea of what this "Empire" meant has been a contentious debate for a long time.

First, because a lot of people in the 19th and 20th Centuries used it to justify national antiquity or the early existence of a state. Second, because there's a lot've we lost from the period 600-1050, and the few sources we have are mostly Church texts and letters, which give a very biased account of what concepts like "Empire", "King", "Aristocracy" or even "Scots" or "Spain" meant. In the Early Middle Ages the Church seems to have been in a process of "culture builidng", trying to bring on a cultural revolution of sorts, getting a Christian society and a Christian world seep into the minds of the kings and lords of the post-Barbarian Invasions world... a world which, as shown by Dumezil in his book "Les Royaumes Barbares en Occident", was much less Barbarian and much more Roman than traditional historiography has expected.

In the case of Spain, the imperial title itself was used often during the 10th Century. It is unclear whether it was supposed to be a claim over the Dominium Mundi the Roman Empire usually signified, or if it was meant as a "collection of kingdoms". Hispania was perceived as bigger than one kingdom, not just because there were several Christian kings (after all, the Visigothic kings had ruled over the whole of the Peninsula, and they had not claimed the mantle of Empire), but also Hispania being a Roman dioceses, a geographic unit of some sort with special significance. The title Magnus Basileus was also used by some kings in León (Ramiro II), which harkens to an imitation of Byzantine example, rather than a recreation of Roman tradition.

What the Imperial title meant in practice was... not a great deal. Princes and kings from Spain paid homage in León, but their duties seem to have ended there. León never had the capacity to really assertain their claim to Spanish preeminence, and the Iberian princes seem to have followed the Leonese claim temorarily because it made it easier to band together to fight the Moors, without actually having to sacrifice anything in return.

There's also the matter of the Cluny propaganda. This theory, which gained traction recently, defends that much of the political and cultural changes between the year 1000 and the 1100's are a result of the Cluny Order trying to strengthen the social and political ties of Christian Europe under the Pope's direct authority. The Pope and the Abbey thanked monarchs who showed obedience to Rome with support from the Abbey, Papal dispensations and copious amounts of propaganda... which also can lead one to think that our impressions on how strong were these Imperial claims may have been greatly exaggerated, especially when most of our sources are clerical, and most of those were copied in Cluny monasteries. But that's my own interpretation on the whole matter, but considering how fast these imperial titles vanished, it's a possibility.

For a short while there was even an "Emperor of All Galicia", and the Imperial titles themselves changed format and meaning enormously during the two centuries they were used intermitently in. Alfonso VI of León seems to even have used the title " al-Imbraṭūr dhū-l-Millatayn" ("Emperor of the Two Religions") in a letter to Al-Mutamid ibn Abbad, Emir of Seville.

How much of this is propaganda and how much is based on real, actual recongnition and sovereignty is up for debate, but most scholars agree that these titles were a sort of early Feudal experiments in sovereignty, before the "One Pope, One Empire" doctrine got reaffirmed under Pope Gregory and later under Innocent III.

So, I'm not sure about the case of Brian Boru, but consider: who wrote the Book of Armagh, why, and what intentions they may have towards Brian Boru?