The Old Crosshaven Circular Trail-– Cork City (the Marina area), 9.1 miles long, mostly follows two former railroad rights-of-way, and cuts through several parks. Absolutely flat. To get there, driving is the best way–trail head is across the river from the Cork Kent Station, which is about a 30 minute walk.
https://www.alltrails.com/trail/ireland/county-cork–2/old-crosshaven-railway-path

Harty’s Way to Passage West Greenway– 5.5 miles long, follows the Passage West Greenway. The Southmost end of the greenway is a 20 minute walk from the Passage West pier of the Passage West Ferry. The northmost end is in Rochestown, where it continues on across an old railway viaduct to the Crosshaven Circular Trail. https://www.alltrails.com/trail/ireland/county-cork–2/harty-s-quay-to-passage-west-greenway

Combined: Passage West Greenway ride-– (this is about 18 miles long, ideal for a bike ride): Ride south from Carrigaloe to the Cross River Ferry. Take the ferry to Passage West. Make a right–head north and ride about 1 mile to the start of the Passage West Greenway. Stay on the greenway for 10 miles until you wind up in Cork City, and take the train back to Carrigaloe.

List of 10 best Cork Ireland Walks— From Ireland Before You Die. https://www.irelandbeforeyoudie.com/the-ultimate-10-best-walks-in-and-around-cork/

19 Walks in Cork— From the Irish Road Trip. https://www.theirishroadtrip.com/best%E2%80%8C-%E2%80%8Cwalks%E2%80%8C-%E2%80%8Cin%E2%80%8C-%E2%80%8Ccork%E2%80%8C/