The Irish once tried to colonize Britain.

For example, in the 5th century CE, there was a battle between Welsh King Cadwallon I and Irish King Serigi Wyddel—which drove Irish settlers out of the island of Anglesey.

Irish invasions may have been a factor in why the Romano-Celts hired the Germanic tribes as mercenaries.

https://en.wikipedia.org/wiki/Cadwallon_Lawhir