"いらっしゃいませ" "いらっしゃい" has more feeling of "invinting you in." (that's why it's used for customers)If someone is coming to visit your house, you can say "いらっしゃい" too. "ようこそ" is Welcoming and also, makes it sound bigger deal than いらっしゃい。"Welcome to my house!" (we don't say that too often unless if it's a party or something important right?) You would hear いらっしゃいmore often in every day life.

To me, ようこそ means welcome in general. You can say XXXへようこそ, where XXX can be a restaurant, a hotel or a place (even if Japan itself). いらっしゃいませ is only for business use, i.e., a clerk in a store or a waitress in a hotel. It's used to welcome the customers when they walk in.