Would you use this to refer to the master bedroom?
Yes, you could do.
Tapadh leat a charaid!
Shouldn't it be: Seo an t-seòmar-cadail mòr? I ask since ... both are accepted; is the t- optional?
I don't think the variant with 'an t-seomar-cadail' should be accepted because you seem to only add 'an t-' when the word following begins with a vowel like 'an t-uisge'.
Somewhere, somehow, i thought I learnt that "seòmar-cadail" was feminine? Now i seem unable to un-learn it.