"Where is the entrance?"
玄関 is a specific area in a Japanese house. It's not the entrance itself, but rather the entrance hall or foyer where you are greeted, leave your shoes, and properly enter.
入り口 is an actual entrance.
If you need to find the way into a building, you would use 入り口. If you get lost in someone's home and need to find the front door, you'd use 玄関.
No. A 玄関 is closer to a foyer, but not a full hallway. It's like a little nook right after the front door, usually set a little lower than the actual house floor. You leave your shoes in the 玄関 and your host usually puts a pair of slippers down on the house floor for you to step into.
Pictures might explain it better. In a traditional Japanese house, it might look like this: http://kesterhouse.com/interior/photos/j_genkan_01.jpg
See where all the shoes are? That's the 玄関.