An interesting chart based on OECD data:
The above captures data for 2010 latest so we can expect the 'Latest' metric to come up as the crisis and rising tax burdens continue to push more and more activity into the Black Economy. Still, at ca 15% of GDP in Shadow economy, the problem of extra-legal economy is non-negligible. Another point to make is that since Shadow economy does not apply to the MNCs activities, Ireland figure should be adjusted for GNP/GDP gap. Which would put us right at Sweden's level. Performing the same for mid-1990s figure implies that Ireland's Shadow economy has declined over the period covered by lower percentage points than any other economy (save Austria's, US' and Germany's - where Shadow economy share rose).