Carrickfergus is generally quite well covered on EE, with mostly 1800 MHz from numerous sites. There is also 800 MHz available from one site near the town.
Some of the other networks in Carrick, and Northern Ireland in general, seem to have more sites deploying 800 MHz than EE do. The trade-off here is between their better perceived coverage in terms of "bars on screen", but this can result in a worse experience for mobile data in the form of slower data speeds and less capacity. EE have chosen to go the other way with it, with a higher band coverage layer and then some of this 'extended' coverage on selected sites.
With EE in NI, generally I have found when you have any form of coverage it will work (with the exception of some areas on the edge of 800 MHz-only coverage). With some of the other networks, you could have 3+ bars but yet still have slow data speeds and issues with calls.
One indoors edge-case scenario is not enough to judge the quality of an entire network unfortunately, and everyone's demands from their mobile do vary.
