To "take the King's Shilling" in England was to enlist as a soldier in the army, once upon a time. The shilling, equivalent to £0.05 in modern currency, was an inducement to enlist.
In Ireland, of course, the English King was not recognised/acknowledged.
So when an Irish man enlisted in the English Army, he was said to have taken the Saxon Shilling (with the implications Waylink suggested).
The term is still alive today, used with reference to the suggestion that the British Government is seeking to secure co-operation of Irish Republicans with promises of government money.
A little off-topic but I hope you'll find it interesting.
You can still find beer tankards made of pewter that were often used in pubs. Some have glass in the bottom and it was explained to me "That's so you can see if there is a shilling in the bottom before you take the drink from the "generous visitor in the pub" because he might be a recruiter. If you "took the shilling", you just unwittingly enlisted , so the glass lets you check to make sure there's no shilling at the bottom. Possibly some truth to the story