Poyning or Poynings? 

It is absolutely and without a doubt Sir Edward Poynings. See the Oxford Dictionary of National Biography entry by Steven G. Ellis, ‘Poynings, Sir Edward (1459–1521)’, Oxford Dictionary of National Biography, Oxford University Press, 2004 accessed 2 March 2008.  As such, the law 10 Henry VII c. 9 should be, and generally is, described as Poynings' law.  For examples see, R. D. Edwards and T. W. Moody, 'The history of Poynings' law, part I, 1494-1615',  Irish Historical Studies, II (1940-41): 415-16 or C. I. McGrath The Making of the Eighteenth-Century Irish Constitution. Government, Parliament and the Revenue, 1692-1714 (Dublin, 2000).  That said, it is listed as Poynings's law in the index of David Dickson, New Foundations: Ireland 1660-1800 (Dublin, 1987), 246.