The Blackstairs Unit is the southernmost of the five units which comprise the Leinster Granite. Four granite types are recognized, which in order of decreasing age are: Graiguenamanagh Granite, Porphyritic Microcline Granite, Non-Porphyritic Granite and Type 1 Granite. Using whole-rock geochemical data, in conjunction with field and petrographic evidence, a model is proposed whereby partial melting of a Lower Palaeozoic immature sedimentary source produced a parental granitic melt which crystallized without further differentiation to form the Non-Porphyritic Granites of the northeastern end of the pluton. At the SW end, this parental melt fractionated to form the Porphyritic Microcline Granite leaving the melt depleted in K2O, which then crystallized to form a slightly more acidic Non-Porphyritic Granite than that at the NE end. A mixture of restite minerals plus melt from this partial melting process crystallized forming the Graiguenamanagh Granite. Fractionation of thorium into restite allanite in the Graiguenamanagh Granite accounts for the low Th and Th/U levels which characterize the Non-Porphyritic Granite and the Leinster Granite in general. A high degree of partial melting of similar source material produced the low volume Type 1 Granite magmas which were subsequently intruded along faults. A similar petrogenesis is envisaged for the whole of the Leinster Granite.