His adviser, history department professor Mary Donovan O'Sullivan, suggested to him that "Ireland's contribution to the military activities of the English crown in the thirteenth century might be a subject that would repay investigation."
[citation needed] Lydon moved to England to attend the University of London and studied under the influential medievalist Sir Maurice Powicke.
[5] Having a full year left on his Travelling Fellowship after his studies were concluded, he was advised by his mentor Powicke to "use the residue of the funding to travel on the continent...stay clear of archives...read, visit galleries, listen to music, meet people and generally lift his eyes beyond the confines of the [Public] Record Office in Chancery Lane."
This experience "gave him an appreciation of European 'culture' in its broadest sense" and enabled Lydon to "bring the historiography of late medieval Ireland to maturity."
[citation needed] Returning to Galway in 1956 he taught history through the media of Irish and English and, in 1959, moved to Dublin to lecture at Trinity College.