In present day Irish politics, who or what is the "Tanaiste"?

The Tanaiste (current incumbent - October 2019 - is Simon Coveney) is the second most senior politician in the Republic or Ireland, second only to the Taoiseach (Irish for chief) or Irish Prime Minister- the current incumbent being Leo Varadkar.

The institution of Tanaiste is relatively recent, only dating from 1937, but the title itself is an ancient one and comes from ancient Irish words for "heir to the chief". However, before the Republic or Ireland achieved independence, the British viceroy was sometimes also referred to by this title.

Although the office, like that of many deputy leaders the world over, is generally honorific, during the recent Brexit controversy Mr Coveney has been a more prominent and outspoken figure.