Rare-element pegmatites have diverse chemical signatures and are important sources of strategic metals such as Li, Cs, and Ta. The two main hypotheses to explain rare-element pegmatite formation are (1) residual magmas from the crystallization of granitic rocks, and (2) partial melts from a relatively rare-element-rich source. In southeast Ireland, spodumene and spodumene-free pegmatite dikes occur along the eastern margin of the S-type Leinster Granite batholith. With indistinguishable emplacement ages around 400 Ma, the origin of the Li-rich pegmatitic fluids has been suggested to have resulted from extreme fractional crystallization of Leinster Granite granodiorite magma. To test this hypothesis, we used whole-rock geochemistry of pegmatite and granodiorite samples from drill cores and geochemical modeling of in situ crystallization and batch melting to investigate which process better explains the formation of the pegmatites. Chemical signatures of the pegmatites and granodiorite do not indicate a direct comagmatic relationship, as the granodiorite has higher concentrations of many incompatible elements than the pegmatites (e.g., concentrations of Zr, Ti, and Y). Concentrations of Li, Rb, Cs, Sr, and Ba show no clear fractionation trends from granodiorite to pegmatite. The in situ crystallization model using the average granodiorite composition as the initial magma generates a range of compositions that does not include pegmatites, so it is unlikely that they represent residual granitic magmas. Modeling of partial melting indicates that the Leinster Granite granodiorite and pegmatite magmas could have formed in separate events and from chemically different source rocks, with the pegmatite magmas presumably formed in a younger event because the pegmatites intrude the granodiorite.