I quite often do a bit of lounge hopping if I've got time to kill in the airport and have never had any problems gaining access to another *carrier's lounge (though there are some exceptions reported by others).

It's all covered on the * website:

http://www.staralliance.com/en/trave...s/lounges.html

Star Alliance Gold card holders and one guest, have access to any lounge around the world that is identified with a Star Alliance Gold lounge sign when departing on a Star Alliance flight, no matter which class of service they travel in. Just look for the Star Alliance Gold sign outside the lounge and present the ticket for your same-day Star Alliance flight, your boarding pass and your frequent flyer programme card featuring the Star Alliance Gold logo. Guests must enter the lounges with an eligible Star Alliance Gold member and must depart on a Star Alliance member airline, but may travel on a different flight.

The map of Dublin is here:
http://www.staralliance.com/int/int_..._map_large.gif

And you'll find general info on the BMI lounge here:
http://www.flybmi.com/bmi/en-gb/plan...acilities.aspx

Enjoy your trip.