A successful colonisation of Leinster, while the plantations of Ulster fail or did not happen as OTL?
One of the reasons the Ulster plantation survived in OTL, while the other's dwindled away (and there were attempts to set up plantations throughout Ireland) was due to the geographic closeness of Ulster to Scotland. There are stories of Ulster Scots rowing over to Scotland to attend Sunday services in their homeland. This allowed for the planters to maintain close cultural, economic and familiar ties to their Scottish homeland, recruit more settlers, and also get assistance in case there were problems.
Leinster just doesn't have those benefits - its not as easy to reach, and its going to have a higher population of those who are going to be resistant to the plantation. In addition to the native Irish, the planters are going to have to deal with the Old English as well. Another reason that the Ulster plantation took off was because there was a large amount of land who's owners had fled the country following the conclusion of the Rebellion of 1641 and the Flight of the Earls. This same situation couldn't be replicated in Leinster, which did not see the same flight of land holders from the realm.