Re-fronting has probably always been there as an option when decisions about renovation, or rebuilding, were being considered, as it is today, but I suspect that the trend to re-front 'Dutch Billys' as flat parapeted 'Georgians', as well as reflecting a desire to conform to the prevailing architectural orthodoxy, involved the other motivation of simple practicality. Exposed projecting pediment topped gables must have been particularly prone to severe weathering and/or occassionally falling into the street.
Whereas Billys survive today in their Georgial altered form all over Dublin, It's interesting to note that where Billys survived long enough to be protographed in their original unaltered state was only in the depressed areas of the city where the houses had become tenements and no investment in the building fabric was being made. This is why 'Dutch Billys' have long been associated with the Liberties in particular, when in fact there are far more Billys (albeit re-fronted) to be seen today in Dublin 1 and 2.
South Fredrick Street is a case in point.
Of the 10 surviving houses on the east side of the street, probably 7 or 8 of them are essentially 'Dutch Billys' built between the early 1740s and the mid 1750s.
The east side of the street looking north. At first glance, this just looks like a normal small scale Georgian Street.
The same terrace from the rear. The cruciform roofs, single shared central chimney stacks and gabled return structures are all classic 'Dutch Billy'. The lengths to which the Georgian owners of no. 27 went to modernize their 'Billy' included not just replacing the whole cruciform roof and front gable, but also the back gable and they even re-roof the tiny return to get rid of it's rear gable.
(There's a nice piece of original timber panelling to be seen in the front room and hallway in Brubaker's Cafe at no. 22)
No. 24 with the very low rain water outlets giving away the profile of the axial volume of the cruciform roof behind the altered flat parapet. The quite unGeorgian window proportions, particularly on the second floor, are also interesting.
South Fredrick's Street in Rocques' time (1756). The street was a slightly lower status version of Molesworth Street, where curvilinear gabled house of all sizes must have presented a stunning contrast to the sober palladianism of Leinster House.
Because of the largely convincing 'Georgian' appearance of the street frontage, it could be argued that the South Fredrick Street terrace were 'transitional' houses that utilized a 'Dutch Billy' plan and rear elevation, but were always flat parapeted and hipped roofed to the front, but I don't buy that. 'Transitional' houses are a distinct group (more endangered even than 'Billys') and there's no evidence that there was ever a house type in Dublin that was designed to be hipped roofed to the front and gabled to the back.
These are photographs from 1995 of a terrace of 'Transitional' houses on James' Street (one of which still survives, just).
The steeply pitched roofs and shared central chimney stacks survive from the 'Dutch Billy' tradition, but there is no exploitation of the attic space and the reduction in window height towards the top storey is original to the construction and 'Georgian' in character. The brickwork is also a mottled yellow/red as opposed to the deep red that is characteristic of the 'Billy'.
As with the three 'Transitional' houses at the west end of Hendrick Street, the flat parapet and hipped roof to the front of these James' Street houses is reflected in an even simpler hipped roof arrangement to the rear. The return is still there, but it's litterally starting to disappear, no longer an essential element, it only reaches to the first floor, or even just the ground floor in some instances.