The Catholic website www.rosaryqatar.com might be really outdated. I called a priest listed there once and it turned out he was giving me directions to his new chapel... in Canada!

Anyway - from what I found out, mass is held near the Marriott daily @5:45am and 7:15 pm and Sundays at 5pm and 6:15pm. You can contact them by phone: 442 6831