How to Extend Wireless Coverage?

I live in an old two storey house and the wireless router is located in the downstairs sitting room. However I'm unable the use the internet in my bedroom, which is located upstairs and at the opposite side of the house, as the wireless does not connect that far. In fact, the internet doesn't even connect in the kitchen, which is the room directly beside (apart from the hallway) of the sitting room where the router is located. I think it may possibly be due to the fact that it's an old house with thick walls.

I'm just wondering is there any way I could extend the wireless coverage so I could use the internet in my bedroom.

By the way, it is Eircom Broadband.

Thanks very much.