Hi Chip:
From my understanding, the Irish Free State first tested the French "Adrian" style helmet, which proved unsatisfactory. The Government then decided upon the German design because it was, quite simply, the best.
Germany was first approached as the supplier, but the Treaty of Versailles forbade the export of military hardware. Consequently, Vickers won the contract to produce a copy (albeit inferior). It's interesting to note that a stepped ventilation lug was retained regardless of shell size, even though the helmet was never equipped with brow armor.
With the expansion of the army and reserves, the Irish Government renewed talks with Eisenhüttenwerk Thale, AG in the late 1930s to acquire M35 Stahlhelms. Versailles was no longer an issue, but Anglo/Irish politics were. Cosequently, the British Government undercut the Germans to ensure the Irish soldier once again resembled Tommy Atkins.
Chas.