From someone that I know over in N.I the reason why Go Ahead went over to ADL and Ulsterbus/Bus Eireann are looking at ADL for their single deckers is down to the fact that Wright are more than happy to build Gemini's for anyone at standard price as a lot of the parts are interchangeable with the Streetdeck, however, whenever an operator has Single Deck needs and wants a batch of Eclipses, they immediately get told they can have Streetlites instead. When that company insists on Eclipses, Wrightbus up the price by quite a bit so the operator then has the choice between staying loyal to Wrightbus and going for the inferior Streetlite or going elsewhere.
In Ireland public bodies and agencies cannot approach manufacturers and ask them to build them something, they are required to undergo a tender process which manufacturers can then put in bids for and the winning tender gets the contract.
These tenders for publicly funded bus services,, are not run by the operator, rather they are run by the National Transport Authority who owns the vehicles and allocates them to operators who are contracted to run services on it's behalf.
What you say about Wright not wanting to build Eclipses and instead offer the Streetlite is very true when it comes to selling on the open market, but only approx 1% of city bus vehicles are bought on the open market in Ireland as everything else comes through a tender.
For tenders, it's normally the chassis builder who puts the bid forward who subcontract the body side of things to the likes of Wright. For integral vehicles the integral manufacturer will put in a full bid. Most of these tenders require a certain level of support network and maintenance and spares contracts, which is where ADL fall down in Ireland.