Sovereignty is one of those concepts that is used in similar, often overlapping, but also rather different ways by different people in different disciplines. From an international law perspective (and I know little to nothing about international law, it’s a whole different sphere of scholarship), sovereignty will be understood quite differently from how it is understood from a constitutional law perspective or from a political theory perspective – and political theorists differ in accordance with their ideology.
I tend to follow Rousseau’s view of sovereignty, which almost completely equates sovereign power with constituent power. By that definition, the Irish people are sovereign because they possess constituent power (constitutional amendments in Ireland require popular approval in a referendum), whereas, say, the Canadian people are not sovereign because they do not possess constituent power (constitutional amendments in Canada can in most cases be approved by a combination of the federal and provincial Parliaments).
This is elision of sovereign and constituent power a rather strict use of the term and many would disagree with it. Some might say, for example, that although the constituent power in Canada is held by representative bodies, those representative bodies only have legitimacy and authority because they act on behalf of the people who retain ultimate sovereignty. However, that use of the term means that sovereignty ceases to have much meaning as a matter of constitutional law, even though it might have moral and political resonance.
It is a common claim today, especially amongst supports of Scottish independence, that ‘the people in Scotland are sovereign’. That’s a moral and political claim. It is a claim with some weight behind it – being asserted by the Constitutional Convention in the Claim of Right in 1989 and reaffirmed by the Scottish Parliament in 2012. But it is not a legal claim. If we had a constitution that placed constituent power in the people (by which I mean the power to make and approve major changes to constitutional law), then that sovereignty would be given reality as a matter of constitutional law; until then, it is just an aspiration.
The connection of sovereignty to constitutional law – and again, I am following but developing Rousseau here – means that sovereignty is always vested (legitimately, as a moral claim which needs to be realised through the constitution) in the people, and never in an individual person. An individual may have rights that are constitutionally protected – rights that are recognised and enforced by ‘the people’ in their sovereign, constituent capacity – but the individual, as such, cannot be sovereign. Each citizen is an equal part of the sovereign people, but a person cannot be sovereign without there being a complete destruction of the res publica. [Of course, there are some folks who are into the idea of the ‘sovereign freeman on the land’, which is a sort of bastardised hyper-libertarian version of Lockean natural rights theory combined with an ahistorical reading of early common law, but it’s complete nonsense.]