Translation:Tìoraidh a thidseir.
The a is the vocative particle, used when addressing someone, except if the name/noun starts with a vowel sound (e.g. Iain, ollaimh "professor", Fhionnlaigh with silent fh-, etc.).
It's like the o in old-fashioned/Biblical/stylised English o great king, o ye of little faith or what have you, but in Gaelic it's in regular use.