Thanks for raising this question, as to the origin of the "Walch" family arriving with the Williamite forces into Ireland. I knew little about the Dutch Prince William of Orange and his battle with James VII for the British throne, and the battle at the Boyne River fought on Irish soil. Your question piqued my interest in how this critical situation affected the (native) Irish folk.
It's interesting, also, to read historians accounts of William's opinion of the quality of his forces: Not impressed by Scottish & English soldiers, his preferred infantry were professionals sourced from Denmark, the Netherland and supported by a large coningent of (French) Huguenot soldiers.
I can see the similarity of Walsh and Walch with modern eyes, but I see a Dutch/Bavarian link there as Walch (and Waelch). The 'ae' changes the vowel sound, and is printed as "a" with dots above -- call umlaut. I am thinking that your Walch could have originated as the trusted, prized professional soldiers selected by Prince William. If that were the case, the UK historians may have record of these hired musketeers. It's worth a try. Particularly the grant of Irish land to the victors at Boyne would/should be recorded somewhere by the British. So that's another source to try. The pronunciation of WALCH in modern German is not like Welsh or Walsh. The W is like our V, the A is a long aahh and CH can be hard like Kick, or soft CHeese. I can't speak for Dutch.
The normal (very common in Ireland) has a meaning of surname WELSH, you can find via online search.
It's also interesting to wonder when the Walch family left Co. Limerick, and why?
