Yes these roofs have become very common in recent years on large-scale restoration projects. We forget what a nightmare it can be for large parts of the year with the Irish and British climate trying to undertake major roofing works that can take many months. Not only do temporary roofs make working conditions much more bearable, they can also reduce costs, or at least be cost-neutral, in eliminating time lost through inclement weather. In the picture below, Robert Francis Architects in the UK finished the re-roofing of this Methodist church four weeks ahead of schedule.
© Francis Roberts Architects
You can clearly appreciate the comfortable workshop-type environment created inside.
On a more prosaic level, they can also useful for assuring completion of works within a timeframe for drawing down grant funding.
Castletown House was probably the first restoration project in Ireland to make use of such a structure on a massive scale. It was particularly useful in that case, as with similar projects, in the complicated field of rebuilding parapets, re-setting masonry urns, cornices and chimneystacks, as well as the more typical leading and slating work. With the battering Westport House gets, it's no wonder they went for this option! I imagine the the sides were later clad in translucent tarpaulin.