Abbreviation of いらっしゃって. A polite/honorific form (te-form) of 'to come / go / be', often used to invite or request someone to come or to state that someone is present.
どうぞいらして下さい。
Please do come.
明日ぜひいらしていただけますか。
Could you please come tomorrow?
先生はもういらしているそうです。
I hear the teacher has already arrived.