In Richard II 2nd expedition into Leinster in early 1399 against the Leinster King Art Mór Mac Art MacMurrough Kavanagh, instead of Art offering the retreating Richard's starving army a treaty what if he had of continued the harassment and killed or captured the king of England. I know Richard was overthrown later the same year by Henry the IV but this event still would have caused some serious butterflies especially if Richard was captured not killed.
In Ireland I reckon either one of two things might have happened.
1. Art with this his confidence would have grown even more and probably would have claimed the title High King of Ireland causing major conflicts between him and the O'Neill's, O'Brien's and any other claiming dynasties, although he had a minor alliance with the O'Brien's of Thomond. Even more he might have claimed Kingship of England as he had the most powerful hostage, but would not have been able to enforce this.
2. The Irish chiefs with this victory push their borders and go into all out rebellion forcing the Hiberno-Normans out or into complete submission and the english towns the same.
I would like people's opinion on this and what England's reaction would be my history of England is not very strong so I can't make a theory, but taking in mind if they did raise an army and came to Ireland Richard II first expedition into Leinster in 1394-95 he had an army with him larger than any English army brought to France during the 100 years war and although he got most chiefs/kings to submit it wasn't a great success.