When did roundhouses fall out of vernacular use?
The time frame we have is full of gaps:
- They fell out of use in South-West Britain in the 8th Century.
- Wales may have had them potentially for longer.
- Rectangular houses become popular among Irish elite 900 - 1000 CE.
What about lower classes in Ireland?
How much longer did they last in Wales?
What about Northern Britain and Scotland?
Roundhouses are a staple feature of Antique and Late Antique Britain and Ireland in the imaginations of most people, their association with the Brythonic and Goidelic speaking peoples of the isles can't be uncoupled. But the later medieval world is dominated by rectangular housing across Europe, I'm interested in the chronology of this change in Britain and Ireland.
During my research, I found that this question is relatively understudied in Britain as compared to Ireland, in Ireland we see a broad consensus that rectangular houses began to replace circular in the periods of 900 - 1000:
We can confidently suggest that by the end of the 10th century the transition was complete ,
Towards the end of the early medieval period (tenth to eleventh centuries), rectangular houses built in stone or turf were normal, and round houses became rare. 
One is the well-known change of the predominant house design from circular to rectangular, which more-or-ess coincides with the period of transition from Old to Middle Irish around AD900. 
These sources somewhat tentatively propose the cause of this change on Norse influence which coincides with the change:
The motivation for this change has been much discussed, but no consensus has yet emerged. One possible explanation is that while the rectangular houses identified so far are not much larger than their round predecessors, they may be a product of elite emulation and that elsewhere, particularly as kings began to interact with, and adopt aspects of, Scandinavian culture, royal halls were taking on a new form and function. 
Of course, if this is the case, can the same time frame be applied to the non-elite of Ireland? Surely a taste for rectangular buildings must have taken some time to 'trickle-down', especially in remote regions and groups that may not have the resources to rebuild their dwellings?
The sources for Britain are even more oblique, they somewhat suggest that roundhouses survived as a vernacular building into, and in some case beyond the sub-Roman period in western Britain, but how far beyond is a mystery:
Round and rectangular houses have been found within British hill-forts reoccupied in sub-and post-Roman times ... 
an un-referenced statement on Wikipedia:
Roundhouses were the standard form of housing built in Britain from the Bronze Age throughout the Iron Age, and in some areas well into the Sub Roman period. 
The Wikipedia entry for Anglo-Saxon Architecture suggests they survived up to the 8th century in South-West Britain and perhaps later in Wales:
Characteristically circular buildings  as opposed to rectangular, often in stone as well as timber, along with sculptured Celtic crosses, holy wells and the reoccupation of Iron Age and Roman sites from hillforts such as Cadbury Castle, promontory hillforts such as Tintagel, and enclosed settlements called Rounds  characterise the western Sub-Roman Period up to the 8th century in southwest England and continue much later in independent Wales at post-Roman cities such as Caerleon and Carmarthen.
But this has really been the limit I've come to in my research, very broadly roundhouses fell out of use in south-west Britain as late as the 8th century, and Wales perhaps later, rectangular houses became the norm for elite Irish 900 - 1000.
This, of course, leaves gaps, when did they fall out of use in Wales? How long did it take for rectangular buildings to become popular through all classes of medieval Ireland? What about Northern Britain? What about Scotland, where the population didn't experience the same continental influences/incursions in the same way or time?
 Early Christian Period Domestic Structures: A Change from Round to Rectangular Plans?, C.J.Lynn, 1978
 Early Medieval Houses In Ireland: Social Identity and Dwelling Spaces, Aidan O'Sullivan, 2008
 The court poet in early Ireland, Alex Woolf, 2013
 Roundhouse (dwelling), Wikipedia, Accessed 04/10/2018
 Medieval Devon & Cornwall; Shaping an Ancient Countryside, Ed. Sam Turner, 2006
 "Flying Past - The Historic Environment of Cornwall: Enclosed Settlements". www.historic-cornwall.org.uk. Retrieved 1 April 2018.