I have read the entirety of the Fenian and Ulster myth cycles, but am asking any fellow historians for good sources of what I guess I'd call practical wisdom and philosophy such as what is in the Havamal?
I think you might be interested in The Irish Triads and The Instructions of King Cormac Mac Airt - English translations of both can be found here: https://celt.ucc.ie/transpage.html
The Triads are a list of maxims. Here are a few of my favourites:
Three slender things that best support the world: the slender stream of milk from the cow's dug into the pail, the slender blade of green corn upon the ground, the slender thread over the hand of a skilled woman.
Three fewnesses that are better than plenty: a fewness of fine words, a fewness of cows in grass, a fewness of friends around ale.
Three silences that are better than speech: silence during instruction, silence during music, silence during preaching.
Three renovators of the world: the womb of woman, a cow's udder, a smith's moulding-block.
Three keys that unlock thoughts: drunkenness, trustfulness, love.
Three tabus of a chief: an ale-house without story-telling, a troop without a herald, a great company without wolfhounds.
Three smiles that are worse than sorrow: the smile of the snow as it melts, the smile of your wife on you after another man has been with her, the grin of a hound ready to leap at you.
The 2nd text is written as instructions on how to be a good king. Both texts give an interesting window into how medieval Gaels viewed the world and what they valued.