In Northern Ireland there are 14 years of schooling (like England and Wales) with 12 being compulsory.
Primary education is named the same as in Scotland (P1-P7).
Secondary school is named either Yr8-Yr14 or in some more traditional schools Form 1-Upper 6th.
The biggest difference is that we still have Grammar schools across the whole country and approx 45% of pupils go to a Grammar school. In the final year of primary pupils can choose to sit transfer tests and their scores on these tests then lead them to applying to secondary schools with their admittance based on the rank order of their scores. Some pupils will choose not to sit the tests or may sit them and not perform as well as hoped - those pupils will go to Non-selective secondaries. We also have Catholic schools and Integrated schools where admittance is based on an equal mix of Protestant, Catholic and none/other pupils.
All secondaries teach the same base subjects and teach towards GCSEs at age 16, but Grammars will tend to go quicker through the material and pupils are expected to be more self-motivated/independent. Non-selective schools will also tend to offer some less academic subjects in older year groups.
The vast majority of schools go through to age 18 with a sixth form included, but some non-selectives stop after 5th form/Yr 12 which is the end of compulsory education. Sixth form colleges are not a thing in NI. If people choose not to stay on for sixth form in school they may go to a further education college (which is very rarely actually referred to as college here) to study more vocational subjects, although some may choose to study A levels there.
It’s worth saying that GCSE and A levels are National Exams. (Our friends in the US are baffled by this concept) There are a few different exam boards (1 in NI, 1 in Wales and (I think) 3 in England?) but they all have to meet certain standards and are all overseen by the JCQ. There is a set curriculum which all pupils follow, and exams are set and marked nationally by external examiners - not by their teachers. Exams take place in May/June each year and then results are released in mid-late August.