Why Dublin?
 

I have been looking for flights from the US (Denver or Newark) to London and the cheapest tickets have layovers in Dublin. Even for airlines other than Aer Lingus. Why?

Because if Aer Lingus decides to offer a low price, others that fly that route want to compete?

Curious. The only airline I know of besides Aer Lingus to fly from EWR to Dublin is CO. What carriers are you showing with the route EWR - Dublin - London besides Aer Lingus?

Continental and Delta have came up yesterday and I thought there was another airline last week that also landed in Dublin before going on to London. Maybe I'm wrong.

Well, the OP doesn't say it has to be one-stop, or same airline.

So, say from DEN, some search engines that are good enough may show US Airways DEN-PHL-DUB connecting to EI to London.

Or from EWR, they can backtrack to ATL or ORD, then DUB.

No US airline fly between DUB and London.

<i>Because if Aer Lingus decides to offer a low price, others that fly that route want to compete?</i>

Bingo.

You are probably finding those deals with Aer Lingus codeshare partners, United, KLM, and British Airways.

Aer Lingus is running a big special right now. You can find great deals on their flights to Shannon, Dublin, London, and Paris right now.
http://www.cheapoair.com/travel/chea...lingus-EI.html

just FYI, as far as I know, Aer Lingus doesn't fly out of Newark anymore :-( They cancelled that route a few years ago - Continental is the main EWR-DUB carrier.