Well, I can safely rule out the WWI story, as I can find uses of the expression that predate the war. A 1912 publication
I do find that most early uses of the phrase "mud in your eye" (with and without "Here's") are in connection with horses and riding, including this amusing cartoon from Punch (1856):
A bit of mud in your eye
Since the 1912 reference (above) in larger context is Here's mud in your eye an' a quick trip back to Texas!, I would lean toward the ideas of speedy travel and leave-taking as being the historical meanings of the toast, rather than health, as suggested by the supposed Biblical reference. "Godspeed" and "May the wind be at your back" come to mind. Clearly, since horseback riding is no longer a primary mode of transportation, the meaning has broadened to include general health and well wishing.
[edit: added the following]
There are also several later references to the toast in the reverse leave-taking circumstance where the speaker is taking leave of the others. In that context it seems to take on the meaning of a parting toast, implying that the mud from my horse will hit you in the eye as I leave or as I ride ahead of you on horseback.
I'm a Pastor preaching this Sunday on a text from the 9th chapter of John's gospel on the healing of a man born blind. To heal the man, Jesus makes mud by mixing spit with saliva and putting it on the man's eye (a popular ancient remedy for vision problems), then telling him to go wash it off. I'm looking for the origin of the toast, "Here's mud in your eye." I'm suspicious of internet claims that it comes from this gospel passage. If it was some typical Irish-type toast that uses a biblical reference, the meaning would seem to be soemthing like, "Here's hoping your vision is restored." Doesn't really seem right.
Other suggestions I've found seem plauysib le but certainly not authoritative, such as: It's a sort of "bragging toast" of the winner of a horse race--since all riders but the winner wind up with mud in their face. Another reasonabel suggestion: toast used by American and British soldiers on leave from the trenches in WWI.
So, any other ideas on the origin of "Here's mud in your eye"? If I hear back from someone in the next day or so, it will likely end up in my sermon, regardless of how authoritative it is! Just want to spice up my intro wiht some interesting stuff.