Aerial photography had been used in archaeology before, notably by St. Joseph of Cambridge, but Leo's name will always be associated with its widespread and, literally first focused use in Ireland. Not only did he discover monuments new to the record; he defined new types!

He equipped himself to do this after his return from the air force by studying early Irish history and archaeology at UCD. Here, he came under the influence of professor Ruaidhri deValera, to whom he submitted a first class masters thesis and to whom he was to remain devoted in his subsequent interests and approaches. The grounding in prehistory always stood to him., especially the emphasis on landscape and total context.

Leo was also influenced by the writings of James Carney, whose Medieval Irish Lyrics.was very important to him. He was especially taken by ’Scel Lemm Duibk’/ ’Winter’ (‘cold lays hold / on wings of bird / icy time / this I heard), which he considered one of the finest nature poems in either Irish or English, comparing it favourably with Shakespeare’s ‘Winter’.