[3] The GS&WR grew by building lines and making a series of takeovers, until in the late 19th and early 20th centuries it was the largest of Ireland's "Big Four" railway networks.

There had been earlier attempts to set up main line railways to the south of Ireland but the 1840s efforts of Peter Purcell, a wealthy landowner and mail coach operator, and his associates were ultimately to prove successful with the implementation of a bill passed on 6 August 1844 for the GS&WR.

The GS&WR's vision to provide a single railway for most of the south of Ireland found favour with United Kingdom Prime Minister Robert Peel as having likely more profitably for wealthy investors and because a single company would be easier to control; these factors likely easing the passing of relevant legislation.

[5] William Dargan, Ireland's foremost railway contractor, built much of the GS&WR's main line and a number of its other routes.

[8] In October 1849 the main line reached the outskirts of Cork, where the GS&WR opened a temporary terminus at Blackpool.

[9] The Irish South Eastern Railway opened between the GS&WR station at Carlow and Bagenalstown in 1848 and reached Lavistown in 1850.

[13] The GS&WR took over the W&CIR in 1900,[14] thus belatedly bringing the rail route between Dublin and Waterford under the control and operation of a single company.

[15] In the latter half of the 20th century Córas Iompair Éireann made this GS&WR branch part of its Dublin – Galway main line.

[17] The branch opened on 2 September 1877 diverging from the main GS&WR line at Islandbridge Junction before tunneling under Phoenix Park to Cabra where cattle sidings and pens were constructed.

A spur from the Drumcondra link line to the DW&WR at Amiens Street was finally realised on 1 December 1906.

[citation needed] In the following years the GS&WR established further hotels in County Kerry at Caragh Lake, Kenmare, Parknasilla (about 3/4 of a mile to the southeast of Sneem) and Waterville.

[22] The British Army was brought in to guard tracks and trains, and Protestant strike-breakers from elsewhere in Ireland to do the work of the strikers.

[26] On 19 December 1916, in foggy conditions, the driver of a ballast train failed to see a red signal at Kiltimagh, County Mayo.

GS&WR routes remain some of the most heavily used in Ireland, linking Dublin with Limerick, Cork, and Waterford.