The geals are part of the interpretation of an age old Irish oral tradation and mythology written down by early christian monks when we learned literacy, with a little artistic licence to put it lightly. The last of five in the invasion myths who up until recently among all the scholarly types were accepted as the celts who imposed thier culture and way of life on the island.
Lingistic people tell us our language is celtic, our art, culture and Gods were celtic. The only problem is there has been no archaeological evidence to prove these invasions.
So the new modern argument for celtic Ireland thats the buzz is that we were not invaded by an army of celts at all, but became 'celtic' in most ways as result of cultural exchange.
This is just a theroy which i haven't really read up as much as i should, so you should do your own homework before taking what i say as gospel.
On that vien of ignorance, the only thing in my mind that could consume and eventually dominate the current Irish culture of that time, if not war, would be have to be religion.
If its right I think it shines a lot of light on what we know of the celtic civalistion from Roman and Greek sources who class them as mere barbarians and shows that they had indeed a very powerful cultural pull which obsorbed those who they came in contact with and would explain how thier influence spread so far and stuck, when all they were was a loose gathering of tribes of most probably multi-ethinic origins.
The English, or anglo/saxons, have been recorded in history as germanic invaders into 'celtic' Britian. The main sources for this info is Gildas, a close to contempory British (welsh) histrian of the time and Beede (anglo/saxon), who wrote the anglo/saxon cronicles a few ages laters on.
Some modern English historians like to point out that there is no evidence of this invasion either.
Like i said, I haven't read enough to really argue. Truth is i doubt you can read enough to be really sure, so its basically up for endless arguement and at best a matter of opinion.