1. This is an example of an expression that has altered its spelling over time. "Not for donkey's ears" means not for a very long time and refers to the length of a donkey's ears. However, they gradually stretched from the "donkey" and turned "ears" into "years". The meaning of the expression is the same, if it does make rather less sense than the original.
2. Because donkeys tend to plod along slowly, they take a long time to complete a task or journey. Therefore, a donkey year takes a long time to elapse.
Fogey/fogy /fougi/ sl. (early 18C+, orig. Scot) old-fashioned, stuck-in-the mud.
Person with old fashioned ideas which he is unwilling to change: Come to the disco and stop being such an old fogey!
You think me an old fogeyand an old tory, his thoughtful voice said. I saw three generations since O’Connel’s time. I remember the famine. Do you know that the orange lodges agitated for repeal of the union twenty years before O’Connel did or before the prelates of your communion denounced him as a demagogue? You fenians forget some things. (James Joyce, Ulysses. PenguinBooks,1992. p. 38)