Looking through http://www.banknote.ws and the Northern Ireland section, the Northern Ireland Banknote designs, with the exception of Bank of Ireland and Danskse Bank (*Formerly Northern Bank) have changed very little since the 1970's.
The Ulster Bank design is more or less the same as that introduced in 1966/70, albeit with changes in the background graphic design. It's bland, but not unattractive.
The First Trust design is also more or less the same as that introduced in the Provincial Bank of Ireland 1977 series, which to me are the most attractive of all the modern Irish Banknote designs.
The Bank of Ireland design is merely a gradual evolution from the major design changes instituted in 1990.
Danskse Bank (formerly Northern Bank) seems to be a reluctant issuer.
As we are all aware, the Bank of England has signalled its intention to switch entirely to plastic, and reduce the size of its currency notes. The Scottish issuers seem to take a lot more pride in their design in terms of using it to showcase Scotland. The Northern Irish issuers seem to be reluctant. In fact, they were the first to discontinue the £1 note from what I gather, with Northern Bank discontinuing £1 issue sometime around 1977/8 and Ulster Bank around the same time. Provincial Bank of Ireland continued £1 issue until 1984, as did Bank of Ireland.
Allied Irish Banks and Northern Bank were also very quick to discontinue £5 notes, both doing so around 1990.
All things considered, a design and size change is imminent. What will the Northern issuers do?