A term of sectarian abuse used by members of the Protestant Unionist and Loyalist tribe against fellow Scots of Irish and Catholic origin. The word implies that they are descended from people who stayed behind in Scotland instead of going back "home" to Ireland, and are therefore aliens, Others, and not really Scots.
It is used by people who generally have other antisocial attitudes to, to foreigners, black people and gays, for example. They are not big on human rights and equality.
"Tarrier" is indicative of hate speech.
“If a black man is abused, you’re not just abusing the colour of his skin – you’re abusing his culture, heritage, his background.
“It’s the exact same when I get called a Fenian, a pauper, a beggar, a tarrier. These people have a sense of entitlement, or a superiority complex, and all I do is stand up for myself.”
From "Bigots use Neil Lennon as a surrogate victim for sectarianism", the National, 4 November 2018.