The House of Commons (Method of Voting and Redistribution of Seats) Act (Northern Ireland), 1929 introduced first-past-the-post elections for 48 single-member constituencies (including Antrim North).
This constituency was one of seven county divisions in Antrim, so it was smaller than the UK Parliament seat. From 1969 there were nine county divisions in Antrim, but the changes in the vicinity of Belfast did not affect the boundaries of this division.
County Antrim (except for parts of Belfast) is a strongly unionist area. There has never been the slightest chance of a republican or nationalist candidate being elected in a single-member Antrim county constituency, however the boundaries were drawn. Antrim North has not been an exception.
At the Northern Ireland general election of 1929 the new Antrim North division was the scene of a reasonably close Unionist/Liberal contest (less than a 20% majority). However this was the last appearance of a Liberal candidate for the Northern Ireland Parliament constituency, which became an extremely safe Unionist seat for the rest of its existence.