Definition of "shure"

"You see, sir, quite hand in glove with Dashing Jerry; met in the same inn last night--preconcerted, you may be quite shure."

1845, Edward Bulwer Lytton, Night and Morning, Complete‎

"Oh, Mr. Munro, shure here's a card for yees," handing me a lady's card.

1873, Various, Lippincott's Magazine of Popular Literature and Science, Vol. XII. No. 30. September, 1873‎

[…] it's shure as I'm tellin' ye, it's them that has the reference to the immejit surroundin's that has the most of power.

1905, Upton Sinclair, chapter VIII, in The Jungle, New York, N.Y.: Doubleday, Page & Company, published 26 February 1906