Ireland was partitioned in 1922, it had been a single country since at least 1542, with a shared culture that extends back long before that.

Also, saying "Northern Ireland is part of the UK" is over-simplified - not all the laws in Northern Ireland are the same as in Great Britain, (and are likely becoming even more different as this mess unfolds).

GB and NI are the two parts of the UK. GB and NI do not have exactly the same laws.

> And Ireland is an entirely different country,

No, they are currently distinct countries, but their differences are neither "entire" nor "total" - Ireland was a single country within living memory and there is significant support in RoI, NI, and GB for a return to a united Ireland. Presenting them as completely separate entities is misleading.

> Different government, different laws, different currency.

Scotland has all three of those.

The answer is "Ireland is a divided land with only 6 of its 32 counties being in the UK, the other 26 counties being an independent Republic since 1922.

Whilst Northern Ireland is part of the UK and there are shared UK laws, Northern Ireland has an independent legal system to England and Wales, (as does Scotland).

Northern Ireland mostly voted to remain in the EU, but if the UK leaves the EU it brings up the issue of an EU border and potentially re-ignites the Troubles between NI, RoI and GB, which the 1998 Good Friday Agreement ended.

Plus (stuff about DUP)"

The reason this thread exists is because the separation is barely settled and the prospect of an EU border is stirring things up.

Why no discussion of the England/France border? Because those two are completely different countries with no recent ties.

Republic of Ireland and Northern Ireland are not completely different and this is directly relevant to why NI does not have exactly the same laws as GB.

https://dictionary.cambridge.org/dictionary/english/ireland
noun uk ​ /ˈaɪə.lənd/ us ​ /ˈaɪr.lənd/ also Eire
an island containing the Republic of Ireland and Northern Ireland

https://www.collinsdictionary.com/dictionary/english/ireland
Ireland (ˈaɪələnd ) noun
1. an island off NW Europe: part of the British Isles, separated from Britain by the North Channel, the Irish Sea, and St George's Channel; contains large areas of peat bog, with mountains that rise over 900 m (3000 ft) in the southwest and several large lakes. It was conquered by England in the 16th and early 17th centuries and ruled as a dependency until 1801, when it was united with Great Britain until its division in 1921 into the Irish Free State and Northern Ireland

http://www.thefreedictionary.com/ireland
Ire·land (īr′lənd)
1. An island in the northern Atlantic Ocean west of Great Britain, divided between the independent Republic of Ireland and Northern Ireland, which is a part of the United Kingdom. The island was invaded by Celts c. 500 bc and converted to Christianity by Saint Patrick in the fifth century ad. Ireland came under English control in the 17th century and was joined with Great Britain by the Act of Union in 1801. After the Easter Rebellion (1916) and a war of independence (1919-1921), the island was split into the Irish Free State (now the Republic of Ireland) and Northern Ireland, which is still part of the United Kingdom.

https://www.ldoceonline.com/dictionary/ireland
Ire‧land /ˈaɪələnd $ ˈaɪər-/ a large island to the west of Great Britain, from which it is separated by the Irish Sea. It is divided politically into Northern Ireland and the Republic of Ireland. Northern Ireland is part of the UK, and many people there belong to the Protestant religion. The Republic of Ireland has been an independent state since 1921, and most people there belong to the Roman Catholic religion. Ireland, especially the Republic of Ireland, is known for its beautiful green countryside, and is sometimes called the Emerald Isle. Many great writers in English come from Ireland, including Oscar Wilde, James Joyce, George Bernard Shaw, and Samuel Beckett. → see also Republic of Ireland, the

https://www.merriam-webster.com/dictionary/Ireland
geographical name | Ire·land | \ ˈī(-ə)r-lənd \
1 or Latin Hibernia \hī-ˈbər-nē-ə\ island in western Europe in the Atlantic, one of the British Isles area 32,052 square miles (83,015 square kilometers)
Note: The island of Ireland is divided between the republic of Ireland and Northern Ireland.

https://en.oxforddictionaries.com/definition/ireland
Ireland proper noun
1 An island of the British Isles, lying west of Great Britain. Approximately four fifths of the area of Ireland forms the Republic of Ireland, with the remaining one fifth forming Northern Ireland.

https://en.wikipedia.org/wiki/Ireland
This article is about the island in Europe. For the sovereign state of the same name, see Republic of Ireland. For the part of the United Kingdom, see Northern Ireland.

https://www.wolframalpha.com/input/?i=ireland
Assuming "ireland" is a country | Use as an island or an administrative division or a given name or a surname instead

I find people conveniently ignore what they don't want to accept, even after facts are spelt out in detail from every angle, the evidence really rubbed in, they'll still cling to the smallest mote and insist on being right.