So the phantom time theory suggests that 297 years were added to chronicles that never existed in order to fabricate some political claims by creating a forged dynasty.

Now this is where the Tír na nÓg story comes into play. According to the folklore, there was one guy who left Ireland for Tír na nÓg during the time when Ireland was great and Christianity wasn't so common yet. He spent 3 years in Tír na nÓg, and when he returned - Christianity was everywhere and Irish people were small and weak and generally life wasn't good. When that guy went to a monastery - the monastery's documentations suggested that he's been absent for 300 years. So that's the 297 years, then.

Now, here is what I'm thinking:
1. Tír na nÓg wasn't Christian and didn't use Christian calendar at that time
2. Something bloody happened in Ireland and probably in most of Europe and the Church had something to do with that, so it had to censor it.
3. After a bloody revolution - no doubt that Ireland was in ruin and life wasn't good and people were sick and starving.
4. When the dude returned to Ireland, he had lived in royal standards for 3 years in a land with no war and hardships, so it's easy to imagine he has gotten pretty fat. After he fell from his horse, his back did the thing that happens when you're fat and overload your back, so it probably looked as if 300 years of age instantly caught up with him.
5. If Tír na nÓg was indeed a real place located to the west of Ireland - it could have been anywhere from North America to Iceland. Maybe Greenland, if it was really that green back then...

What do you think about the idea?