Will they ever persecute people of Irish (celtic) descent in England?
The law prohibits outright prejudice like signs saying 'No Irish' - but like any other bigotry it continues. Churchill was elected Prime Minister, so I doubt he was any more persecuted than, say, Disraeli.
the celts inhabited many parts of europe. not just ireland the celts are pretty much just ancient europeans that inhabited all of western europe and the british isles
They already have.
The "English" are of mixed descent, as are the people of most parts of the world. And note that at least one Roman legion guarding |Hadrian's wall in the north of Roman "Britannia" was black North African. The postings were long-term, so the soldiers were encouraged to marry local girls and settle down. So you too have a bit of fairly recent African ancestry.