Absence of proof is not proof of absence, theologians like to say. What applies to the existence of God also pertains to historical fact. We know events occurred even if we lack scientific-grade evidence for them.

The point relates to an emotive question in Irish social history: Did the signs purportedly placed in lodgings across Britain 50-plus years ago, “No Irish, No Blacks, No Dogs”, actually exist? And, if so, how prevalent were they?

Raising the issue risks causing some offence. A retired American history professor created a storm a decade ago when he published a paper questioning the extent to which “No Irish Need Apply” (NINA) notices existed in the United States in the mid-19th century. His argument that there was “a myth of victimisation” fast unravelled when a 14-year-old high school student discovered widespread circulation of the NINA phrase in newspaper notices right up to the 20th century.

The story of the Irish in America is, of course, a complicated one. “In the 19th century the Irish were seen as racially backward”, but over time “the Irish began acquiring their whiteness… in part through their role in the enforcement of workplace colour bars against blacks”, writes Kenan Malik in his book Not So Black and White.

As for the Irish in Britain, “complicated” doesn’t do it justice. Anti-Irish discrimination took on many forms over the centuries. However, drawing parallels between such prejudice and the kind of racism people of colour have experienced remains contentious.

Image of a 'No Irish, No Blacks, No Dogs' sign which was donated to an Irish history group in the UK in the 1980s. Photograph: Archive of the Irish in Britain

Regarding “No Irish, No Blacks, No Dogs”, the only known photograph of such a sign is from an unknown source – the........

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