That is absolutely incorrect, St. Patrick converted the Irish Celts in the 5th century almost a hundred years before your date. Further Celts were present in Ireland in addition to the rest of Britain during the 1st century BC
The Celts came to Ireland in a time period more of the range of 700-500 BC.. we aren't exactly sure. They came to Britain around the same time. A bit earlier than 100 BC though.
You originally said
> That being said Welsh and Irish most certainly did not diverge prior to migration to Britain as they would've largely separated when the groups were divided by geography not prior.
The Celts came to Britain and Ireland. Welsh and Irish separated after the Celts arrived to both Britain and Ireland, not when the Celts just went to Britain.