That's a very good question and, no, there's no quick and easy answer. The Annals of Ulster put the Milesian arrival in the 6th century BC, while the Annals of the Four Masters put it 1694BC. Beware of the names - there's more than one Amergin in the old stories. There was an Amergin who was a wise counsellor for Cuchulainn, who I think is the Amergin you refer to from the T?in B? Cuailnge - not to be confused with Amergin Glungeal who came with the Milesians.
The reason the 1694BC date is significant is because astronomically it makes perfect sense. On the day of Bealtaine in 1694BC, the sun was located above Orion, where the sun's path crosses the Milky Way, the heavenly Boyne river - Bealach/B?thar na B? Finne - Way/Road of the White Cow.
Amergin said, "Who but I knows the ages of the Moon? Who but I knows the place where the sun sets?"
The whole area of the interpretation of the Annals is a grey one, but Douglas Hyde, who was a President of Ireland, said that the annals were extremely accurate and that modern astronomers could pinpoint events mentioned in the annals. I must dig out that quote , it's very interesting.
Hope this helps.
Creator and Curator,