Thomastown is the name of the townland which comprises large portions of what is now Sallynoggin and Glenageary as shown on the map opposite.  On modern maps, the area is centred on the Glenageary Avenue, which runs from the Glenageary roundabout towards the Graduate roundabout.

In a valuation completed by James Carolan for Griffiths Valuation dated 10 Aug 1848 the area was shown as 213 acres and 29 sq perches, and this precise area is also shown on maps of the period.

Although the area appears to be dominated by Glenageary house, it was in fact held as a number of separate holdings, the largest of which was 47 acres. Glenageary House was reached by a very long avenue, the entrance to which was just opposite Altadore on the Upper Glenageary Road.  The house itself was located close to where the Sallynoggin College of Further Education now stands.  The house itself was owned by Halliday Bruce(1789-1856), a noted Dublin stockbroker.  The house appears to have been remoddled for him by John Skipton Mulvany.  On Halliday's death, it was purchased by Thomas Pim, Quaker(18nn-1896).  The house was demolished after falling into bad condition in the 1970s, and the Church of Our Lady of Victories is now in the grounds of the house.  Fields surrounding the church were the site of an annual fair and horse show in the 1960s.

Much of the western part of Thomastown was purchased by Dun Laoghaire Corporation for the development of housing estates, and thes included Pearse Park, Drive, Street, Road, Avenue, Villas.  The central line of Thomastown was reserved for road construction for many years before Glenageary Avenue was opened in the late 1990s to join the Glenageary Roundabout with the Graduate Roundabout.  The eastern part of Thomastown is a maze of housing estates.