All placenames that variously show up as tarbert, tarbat or tarbet in their anglicised form derive from either Irish an tairbeart in Ireland and Scottish Gaelic an tairbeart in Scotland, both commonly translated as "the isthmus" today.
Both these words derive from two Old Irish elements, tar "across" and a nominalised form of the verb ber "to carry. The /ɾ/ in tar was assimilated to /ɾʲ/ as a result of being next to the historically palatal /bʲ/ in Old Irish, causing the change in spelling from tar to tair-. So the literal translation would be an "across-carrying". The reason for this is that all tarberts are in fact located at or near old portage sites.
In English language spellings the first syllable "tar" has generally remained constant but the second syllable "bert" has variously been spelled as "bart", "bert" "bat", "bad" etc.