Jenn, I am a volunteer co-administrator of the Sennett surname group affiliated with Family Tree DNA. We believe that Sineath and Sinnott/Sennett/etc. are alternate spellings of the same surname and that perhaps the first person to use the surname was Adam FitzSynad (or FitzSinath) in 12th century Ireland. I elaborate on how I believe our surnames are connected in my Sineath genealogy Facebook page (https://www.facebook.com/groups/452797462405334). We are trying to establish DNA proof of this potential link by using y-DNA, which is passed almost unchanged from father to son. This allows us to establish genetic relationships much farther back that the typical autosomal DNA test. For more information, please check out that Facebook page and if you are interested, please send me a note. [email address removed]