There were people in Ireland, thousands of years before Celtic languages diffused in. What they spoke is unknown. Some have surmised a language or languages related to modern Basque, but this is mere speculation. How a Celtic language arrived in Ireland and the numbers bringing it, is also the subject of speculation - from gradual cultural diffusion, to large scale invasion. The truth probably lies in the middle.
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Shalom/salaam.
10,000 years of Middle Eastern civilisation and the place is not at peace but rather in pieces.