What is the difference between ь and ъ?
Ь - or 'soft sign' indicates the softness of the preceding consonant. Ъ - or 'hard sign', in turn, indicates the hardness of the preceding consonant.
What is 'Pluralia Tantum'?
Pluralia Tantum are Latin nouns that appears only in plural. For instance, nugae, indutiae, praestigiae, Athenae, tenebrae etc.
Can we call the battle of Clontarf 'the liberation of Ireland from Vikings'?
No. First of all, Ireland was never occupied or conquered by Vikings. Instead, Vikings established a few trading posts, later developed into cities (such as Dublin, Waterford, Limerick etc.) and also, especially during the first centuries of so-called 'Viking era' raided Ireland frequently. However, they never had any control over the island. As for the battle of Clontarf, it was a struggle between Brian Boru, the king of Munster, and Mael Mordha, the king of Leinster. Mustermen hoped to submit the Leinster, including the Viking kingdom of Dublin. Despite the fact that Mael Mordha brought a huge number of Scandinavian warriors as allies, it was not the fight between Irish and Vikings. After all, there were as many Vikings in the army of Brian Boru. So, the idea of the 'liberation from Vikings' came from the later Romantic period, when Vikings were considered as 'predecessors' of English men.