Gerald of Wales lived during the end of the 12th Century and was part-Anglo-Norman, part-Welsh and connected (it seems) to most of the important people of court during his lifetime partly because his grandmother had several children by several different partners, some of whom went on to be involved in the Anglo-Norman colonisation of Ireland, some of whom were part of the Welsh nobility. Gerald was a churchman, and was highly educated - in particular he spent a decade in Paris which was one of the première centres of learning at the time. In terms of a clerical career the job he really wanted but never got was to be Bishop of St. Davids - and he wanted to turn that bishopric into an archbishopric separate from the Church in England. Which is probably why his applications for the job were prevented by both the Archbishop of Canterbury (who didn't want the Church in Wales leaving his jurisdiction) and by the King (who didn't want a separate Church in Wales as that might encourage ideas about political separation).
After his return from Paris Gerald worked for the court in England, as a clerk. And travelled around Ireland (with Prince John) and Wales - and wrote books about these travels which are part narratives about the journey, part description of the lands & peoples, and part scholarly explorations of where the line between human and animal lies. This last was a particular theme of Leyser, and she kept coming back to this during the programme. These books, and his other works (including several autobiographical works) were and, in some cases still are, well read. Whilst full of propaganda (portraying the Irish in particular as barbaric because that justifies the conquest of Ireland) they also contain more mundane descriptions of life at the time. And also fabulous tales (like the Bearded Lady of Limerick, or about beavers biting their own testicles off to prevent hunters from killing them).
The conversation on the programme got quite chaotic, although still always easily followable - it had the feeling of a subject that was too full of good stories to miss anything out. I knew of Gerald of Wales before, because my parents have mugs decorated with a Gerald of Wales themed design - I think they must've been bought in 1988 while we were on holiday in Pembrokeshire as that was the 800th anniversary of his travels around Wales. But all I really remembered was that he wrote a book about Wales, so interesting to learn more about the man.