The Protestants aren't just there by chance. They (or rather their ancestors) were planted there by England and Scotland specifically because Ulster was the most powerful and troublesome of the Catholic states in Ireland.
You could make a case for, perhaps, a plantation in another region instead (say Munster) but Ulster had had that leading position for quite a while, so I don't think it's that likely.
If you want random bits of Ireland staying in the Union, there's always the old treaty (with in OTL the Free State didn't go for, partly because of WW2 and the neutrality policy) about Britain retaining naval bases dotted around the south and west coast.