Flying via Dublin

My parents are flying from the US to Manchester via Dublin. They will go through immigration in Dublin and not Manchester, right? What about customs? Will they have to collect and recheck their bags?

Since that is a different country I wouldn't think they would process through Dublin but I learn new stuff all the time. Yes I know it is part of the EUC 
You can go through US customs in airports in Canada and that is weird to me.

Jim, how exactly does your post answer the question? Apart from saying "I don't know"? The airline is the place to ask those questions. (What is the "EUC"?)

They'll go through immigration in Dublin, and their bags will be checked through to Manchester--or should be, depending perhaps on the airline, which one are they using?

American, then switching to Aer Lingus in Dublin. But they're partners, and it was booked as a single itinerary, not as separate flights.

In that case, their bags will be checked to Manchester, and they'll go through Irish immigration in Dublin. 

just checking, but they are only visiting right?  I'm assuming they as much, but thought I'd bring up that there are difficulties with travelling through Dublin with visas.  If they, as I'm assuming, are just visiting you, I imagine you are fine.

In that case, their bags will be checked to Manchester, and they'll go through Irish immigration in Dublin. 

Oh, that's good! They were worried about time, but if they don't have to juggle bags, they should be fine.

just checking, but they are only visiting right?  I'm assuming they as much, but thought I'd bring up that there are difficulties with travelling through Dublin with visas.  If they, as I'm assuming, are just visiting you, I imagine you are fine.

Yes, just visiting for Christmas. I've been through that worry already.

Thanks to both of you!