Ireland as Portugal really is what you want to go for, I agree.
I think that your PoD needs to be a long way back, in the 14th century at latest. The critical element of creating an Irish trade power is a destabilized Great Britain, and particularly a disunited England. The best way to do that, I think, is to have a less thorough Norman conquest - where powerful Saxon lords are dominant in England, with more regular civil wars that break English centralization. If you could also get into perpetual wars of succession - perhaps if, say, William takes the throne, but the Godwinsons survive and the Scandinavian claimants regularly launch attacks - that would do the trick of creating a strong, squabbling nobility that would be difficult to centralize.
Having broken England, what you need to do is create a clear, central state in Ireland, under one dynasty - that dynasty could be Irish, Norman, or Scandinavian, but it needs to establish a strong nation. Ideally, it also messes around in England and keeps it unstable, perhaps due to dynastic ties between the Irish royal family and English nobility. Ireland doesn't need to be perfectly centralized, and can certainly continue to have strong nobles, a feudal government, and its own succession disputes and civil wars, but what it needs to do is be more centralized than England, over a period of time lasting minimally centuries.
By the time of the *Age of Discovery, Ireland is in a good position (especially if it is an Irish explorer who discovers either the New World or - even better - the eastern route to the Indies) to become a significant trade and colonial power, especially if England continues to be weak. Even if England stabilized to some extent, TTL Ireland and England are likely to be on a more equal footing than in OTL, alike to OTL Portugal and Spain. (EDIT: Emphasis on more equal - if England ever really centralizes and unites, it will almost necessarily be dominant in the of the British isles to some extent)
In the long-term though, Ireland just isn't really in a position to maintain power status into the industrial revolution. It simply doesn't have the resources to do so. But it could remain a cultural power into modern times, if Irish-speaking settlers and/or Irish-dominated colonial states in Africa, the Indies or the western hemisphere are well-established, and remain something of a middling power in European power politics.