The people of Ireland are one group with a shared culture and history that is clearly distinct from Britain's
Irish culture is distinct from British culture. The people of Northern Ireland and the Republic of Ireland are culturally identical. They share the Irish language and share phrases within the English language. If sovereign states are determined by national, ethnic and cultural groups, then Ireland should be united as a single sovereign state due to its shared culture.
Northern Irish unionists are a distinct cultural group. They do not speak Irish, they do not partake in most parts of Irish culture and they have a strong culture of their own with its own traditions and linguistic dialect. As a result, Ireland's people are not one nation and are in fact two.
[P1] Sovereign states are determined by cultural, national and ethnic groups. [P2] Northern Ireland and the Republic of Ireland share a single culture. [P3] England and Northern Ireland do not share a culture. [P4] Therefore, Ireland should be a single, united, sovereign state.
Rejecting the premises
[Rejecting P2] Northern Irish Unionist do not share a culture with Irish Republicans. Therefore, they should be distinct sovereign territories.