## 1 Introduction  
Recent advances in interferometric synthetic aperture radar (InSAR) techniques open up the possibility of monitoring ground motions at unprecedented resolution and accuracy, even for terrains that were previously challenging for such techniques. Such advances include more frequent satellite revisit times along with increased coverage, higher resolution of imaging, and ever-increasing computational capacity at ever-decreasing cost. Additional advances include new approaches and algorithms for processing SAR imagery, especially for time-series generation (e.g., [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]) and noise corrections (e.g., [11]) using multi-temporal acquisitions. While historically InSAR has been applied widely and with considerable success to arid, weakly-vegetated landscapes (e.g., [12; 13; 14]), its application to wetter, vegetated regions of the world has generally been more limited. This is especially the case where subtle surface deformations occur at local scales, because of reduced signal-to-noise ratio and temporal and/or geometric decorrelation effects. Such problems are particularly acute in regions of temperate oceanic climate (climate zone Cfb in the Koppen classification [15]), which characterizes much of western Europe and New Zealand, as well as parts of Southern Australia, South America, and North America (Figure 1). Characteristics of such a climate include small annual range of temperatures, constant annual precipitation and high average annual cloud cover. In Ireland, for example, an average cloud cover of around 80% was observed between 1984 and 1994 [16], and an average annual rainfall of 1230 mm was measured between 1981 and 2010 [17]. Land-cover in the Republic of Ireland is mostly vegetated, and only 1.6% of the territory is classified as urban. The vegetation cover is primarily agricultural (68%), followed by peatland (15%), grassland (6%), forest (5%), and coastal wetlands (<1%) [18].  
In this paper, we present a case study of the application of multi-temporal InSAR in Ireland. Our study is the first (to our knowledge) to do so in Ireland with data from the recently available Sentinel-1 satellites constellation of the European Space Agency's Copernicus Earth Observation program. Previous attempts to apply multi-temporal InSAR to Ireland have been mainly based on using first-generation ERS-1/2 and ENVISAT satellite data and have been limited to urban areas, such as the cities of Dublin, Cork, [19; 20] and Belfast [21]. With the recent development of the Sentinel-1 satellites, however, past issues stemming from low resolution in space and time may be surmountable even outside of urban areas. The aim of this study is, therefore, to test the potential of radar interferometry based on Sentinel-1 satellites for the cost-effective and extensive detection of ground instabilities in both urban and rural areas of Ireland. We begin with an overview of known types of ground motion in Ireland, followed by an introduction to the geology and landscape of two study areas upon which our study focused. We then outline the Permanent Scatterers InSAR (PS-InSAR) processing approach [1] used to generate InSAR time series of ground motion in those areas. We then present results showing that the PS-InSAR approach employed with Sentinel-1 data enables the detection of mm-scale ground motions occurring over small areas (\(10^{-3}\) to \(10^{1}\) km\({}^{2}\)) in a country with the aforementioned challenging climate-related characteristics.