"Irasshaimase!" is an honoriffic expression welcoming someone somewhere (usually into a place of business). It shows respect toward the performer of its action (the customer) by honoring them as opposed to humbling the speaker.
The plain form of the word itself is irassharu, and is an exalted form of several verbs including kuru (to come), iku (to go), and iru(for animate objects to exist).
Okyaykusan wa omachininatte irasshaimasu.
(The customer is waiting.)