Taoi·seach (t??sh?KH)n. pl. Taoi·sigh (-sh??) Irish The prime minister of Ireland.[Irish, from Old Irish toísech, first, leading, leader.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Taoiseach (?ti??æx) n (Government, Politics & Diplomacy) the prime minister of the Republic of Ireland[from Irish Gaelic, literally: leader]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014TaoiseachAn Irish word meaning leader, used as the title of the prime minister of the Republic of Ireland.Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited