Ballina, Westport, Galway, Limerick, Tralee, Cork and Waterford intercity services, as well as the Portlaoise suburban services all use that line into Heuston. I'm still not sure there's spare capacity even if they reopened Mullingar to Athlone and moved the Connacht traffic back to its 'spiritual home'. It would make for slower journey times west and introduce congestion for the suburban services east of Maynooth on the Sligo line. I expect it would probably require major additional works too - and not just on the stretch between Mullingar and Athlone.
Don't get me wrong: I think there should be a station in Ballyfermot by the way, and I agree it probably should have been done years ago. I just don't think that it's that straightforward as things currently stand. I'm hopeful that DART+ will bring major improvements and I'm pleased to see DoctorPan's confirmation that a Kylemore Station is in the pipeline.