Intrusions beneath the flanks of Eyjafjallajökull occurred in 1994 and July-December 1999 and were accompanied by increased seismic activity and was constrained by tilt measurements, GPS-geodesy and InSAR.
Studies of these intrusions “reveal sill intrusions at 4.5–6.5 km depth as the most likely source of deformation. The 1994 and 1999 intrusions had inferred volumes of ~(10–17) × 10⁶ m³ and ~(21–31) × 10⁶ m³, respectively” (Sigmundsson et al., 2010 and references therein). 
Between mid-2000 and 2009, earthquakes occurred intermittently at rates of 1–4 events per month, while deformation remained negligible (Sigmundsson et al., 2010).
Sigmundsson et al. (2010) present geodetic covering the Eyjafjallajökull over its most recent eruption that started with an effusive flank eruption of basalt from 20 March to 12 April 2010. The main eruption started on 14 April 2010. Eyjafjallajökull is unique at present in the InSAR record in that it shows multiple short pulses of uplift, two of which did not end in eruption within a year and one that did (Biggs et al., 2014).
Sigmundsson et al. (2010) show that deformation during this period of eruptions can be split into three phases:
“(1) a pre-eruptive stage of inflation due to a complicated time-evolving magma intrusion that produced variable and high rates of deformation, in particular after 4 March;
(2) from 20 March to 9 April, a co-eruptive stage characterized by a pause in deformation (negligible rates); and
(3) a co-eruptive deformation stage associated with the April–May summit eruption, indicating gradual deflation of a source distinct from the pre-eruptive inflation source.”
Parks et al. (2014) present “long-term deformation time-series spanning > 20 years for Hekla, Katla, Eyjafjallajökull and Askja volcanoes, based on a variety of geodetic techniques including InSAR, GPS and tilt measurements”. At Eyjafjallajökull, the authors state that “post-eruption deformation observations reveal inflation, possibly related to the influx of new melt or readjustment of crustal stresses following the 2010 eruption”.
The last historical activity prior to an eruption in 2010 produced intermediate-to-silicic tephra from the central caldera during December 1821 to January 1823, before geodetic measurements (GVP).