It initially had only 612 members, most of whom worked for the Great Southern and Western Railway.
In 1925, it applied to affiliate to the Irish Trade Union Congress (ITUC), but was rejected as other union already represented engineering workers.
In 1933, it renamed itself as the Irish Engineering and Foundry Union (IE&FU).
It reapplied to the ITUC in 1939, but the Plumbers', Glaziers' and Domestic Engineers' Union objected to its membership, a situation resolved by transferring the IE&FU's heating section to the plumbers' union.
However, the merger was not legally finalised until 1976, and the NEU existed on paper until 1977.