Irish Rail says the cuts are temporary and necessary to carry out fleet upgrades ahead of a service boost in the autumn.
Commuters travelling from Newbridge and Maynooth are facing packed trains as Irish Rail reduces capacity on key routes until October for maintenance works.
Morning services like the 6.58am Newbridge to Grand Canal Dock and 7.12am Maynooth to Connolly will now run with just three or four carriages - down from six or more - leaving many passengers standing during peak hours.
Evening rush-hour trains are also affected, with the 5.51pm and 6.22pm departures from Connolly to Maynooth now operating at half capacity.
Irish Rail says the cuts are temporary and necessary to carry out fleet upgrades ahead of a service boost in the autumn.

Kfm Obituary Notices
Kildare Rose Jessica Hunter Set For Rose Of Tralee Festival
Gardaí Appeal For Information Following Maynooth Shop Theft
Missing Person Appeal: Megan Phillipson (34)
Kildare Talent Set To Shine At Ireland's Largest Music Festival Electric Picnic
Over 100 Events Taking Place Across Kildare For Heritage Week