This is the 5472nd most frequent Japanese word.
いらっしゃっ
It is a polite expression meaning "to come," "to go," or "to be (present)."
In this sentence, 'いらっしゃったら' is a polite form of 'いる/行く/来る' applied, indicating the presence or arrival politely.
店にいらっしゃったら是非おすすめの商品を見てください。
When you arrive at the store, please check out the recommended products.
Here, 'いらっしゃってください' is used as a polite request for someone's movement towards the speaker.
どうぞ、こちらにいらっしゃってください。
Please come here.
In this case, 'いらっしゃいます' is used to politely indicate the presence of individuals.