Thanks! Do you mind if I ask where the 'h' come from? In De + Fine = D'fhine, I'm really quite a noob on mutations.

From an Old Irish orthographical convention?

Originally, lenition was only indicated by adding h to the consonants c, p, and t. Lenited forms of f and s were written ḟ and ṡ respectively (and pronounced [0] [i.e. silent] and [h]). Later, both practices were expanded and the convention became overdot in cló Gaelach and following h in Roman script.

Or are you asking how lenition originated?