Signalling and removing level crossings would improve journey time. Having a third track for a few km between Maynooth and Glasnevin would allow fast trains to pass stopping trains. There are already three-four tracks for the distance from Glasnevin to Connolly.
The only scope for additional stations are:
Ballybough (depends on whether the track is straight enough)
http://maps.osi.ie/publicviewer/#V1,716805,735987,7,3
Glasnevin:
http://maps.osi.ie/publicviewer/#V1,715113,736359,7,3
Porterstown:
http://maps.osi.ie/publicviewer/#V1,706068,737773,7,3
There just might be space for a Luas stop in Cabra:
http://maps.osi.ie/publicviewer/#V1,714057,736756,6,3 but access and distance to public roads are issues. It is unlikely the area would ever be developed.