I guess you mean Llewelyn ap Iowerth aka the Great aka Llwelyn Fawr?
I don't think so; like most rulers of Gwynedd, his strength came from the fact he appeared the one Welsh prince who could possibly defeat the English; this was also his greatest weakness-the other Welsh princes might not want to be dominated by England but if anything they wanted to be dominated by Gwynedd even less.
The other big problem is always going to be the Welsh tradition of diving estates between all sons on the death of the father; this includes kingdoms. Now Llewelyn might have been trying to head this off when he had his first son, Grudydd, disinherited in favour of this second son, Dafyd; but with the various other welsh princes wanting their lands free from Gwynedd domaniation again and the Marcher Lords wanting to expand and regain lost lands, its hard to see any Gwynedd domaination of Wales surviving Llewelyn's death.
Plus, Llewelyn took advantage of a situation where England was involved in a civil war; at such times the Welsh lords often make gains but then lose them again when the English sort their problems out. I'm not sure that even if they follow Gerrard of Wales advice that they can overcome this.
My knowledge of early 13th Century Irish history is limited but if I remember aright, the Anglo-Normans controlled the eastern half of the island (Munster, Leinster, Meath and Ulster); the Irish were limited to the West and centre. Unlike the Welsh, I don't think the Irish managed to make that many gains at this time of weakness.
I think really you've chosen the wrong Llewelyn-his grandson, Llewelyn the Last, is a better candidate for some sort of independent Wales
