If I wanted to say someone is bored would I be able to say
メアリイさん は つまらない です
or would this imply that the person is boring rather than bored?
If I wanted to say someone is bored would I be able to say
メアリイさん は つまらない です
or would this imply that the person is boring rather than bored?
It can mean both, but your example is tricky because, as has been discussed here multiple times (e.g. たい vs. たがっている), one often could not directly express another person's feelings as one's own in Japanese.
「メアリーさんはつまらないです。」
in the strictest sense, only means "Mary is boring."
To make it mean "Mary is bored.", one needs to add words and say:
「メアリーさんはつまらなさそうです。」
Not to confuse you, the sentence just above can also mean "Mary seems to be boring."