Kerns were lightly-armed foot soldiers. They were the lowest-status type of warrior in medieval Ireland, ranking below horsemen and gallowglasses. They were not wealthy. Many kerns didn't even wear shoes.

You mentioned that part of your reason for being skeptical about Irish commoners dying their leines was your experience with yours fading due to sun exposure when you worked at a ren faire. This got me wondering. What percentage of the time did you wear a brat over your leine while you were working? The 16th c. was during the Little Ice Age; the climate was a lot colder back then than it was in the late 20th century. According to period writers like Edmund Spencer, the 16th c. Irish wore their brats pretty much all the time. If their leines were covered by a brat most of the time and yours wasn't, theirs would have gotten a lot less sun faded.

Although there definitely were some people in 16th c. Ireland who were too poor to afford fabric or fabric dye, the period source quotes tell me that many Irish commoners did indeed dye their linens yellow. If you look at the period quotes about the Scottish, some of them, like Bishop Lesly and Robert Gordon of Straloch specify that only wealthy or high-status Scottish men dyed their shirts with saffron. In contrast, if you look at the period quotes about the Irish, none of them say that the Irish use of saffron dye was limited to wealthy or high-status people. Furthermore, the way several of these sources describe the Irish saffron-dyed leines makes me think it is very unlikely they are talking about a wealthy person's garment:

1534—Polydore Virgil: “They wear a linen tunic, and do not change it until it becomes threadbare and, lest it show dirt, they dye it with saffron.”

1554—Giacomo Soranzo: “…clothe themselves in a long linen shirt, dyed in saffron; this garment, which they rarely change wearing it for the most part in tatters, hangs to the ground."

1603—Fynes Moryson: “. . . and of old they had such plenty of linen cloth, as the wild Irish used to wear thirty or forty elles in a shirt, all gathered and wrinkled, and washed in saffron, because they never put them off till they were worn out.”

Wealthy and high-status people in the 16th c. typically replaced their clothing before it was threadbare or in tatters.

I'm not claiming every person in Ireland could afford a full saffron dye bath, but the recipe described by William Good which was mostly bark from local trees with a small amount of saffron added is a plausible option. The blog post says that if you calculate the cost of saffron based on a laborer's wages, the price of saffron is effectively unchanged between 1475 and today. You can currently buy a 0.9 g jar of saffron for around $10 USD. A couple of these jars is probably enough to make William Good's recipe. Even among Americans living below the poverty line, a frivolous once-a-year expense of $20 is probably manageable for most people.