US immigration through Dublin and Shannon airports?

I know that Dublin and Shannon offer the facilities to be pre-cleared on flights to the US, but I wonder if they offer ALL the services? My impression was that they can clear people for 90-day visitor visas -- but can they also clear you if you are planning on immigrating to the US? (yes, I know you need to get a... show more I know that Dublin and Shannon offer the facilities to be pre-cleared on flights to the US, but I wonder if they offer ALL the services? My impression was that they can clear people for 90-day visitor visas -- but can they also clear you if you are planning on immigrating to the US? (yes, I know you need to get a visa to immigrate: we have the sealed packet of information, the CD with xrays, and the visa in the passport...)
Many thanks!
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