At the split of the Volunteers at the outbreak of the First World War, most of the Ballylanders company took the Redmondite side, however Crowley, along with a strong minority, chose to follow Eoin MacNeill.

Crowley was subsequently inducted into the Irish Republican Brotherhood (IRB) in 1915 by Ernest Blythe, with his many brothers soon to follow, and later that same year attended the funeral of Jeremiah O'Donovan Rossa at Glasnevin Cemetery.

[4] Kilmallock's postmaster came to Crowley's Drapery in early 1916 with an order from the government for the family to sever their revolutionary connections.

To this, Tadhg's father, Timothy Crowley, gave no definite reply, and he contacted Eoin MacNeill about the matter.

[4] In July 1920, the drapery was attacked by the British, and soon after they came back to arrest Tadhg's brothers Peter, John and Michael, the former two of whom would be part of the 94-day-hunger-strike in Cork Gaol, the longest without any food in history.

During the campaign, meetings across the county in support of him were addressed by a wide range of Anti-Treaty leaders, including Mary MacSwiney, her sister, Annie MacSwiney, Caitlín Brugha, Kathleen Lynn, Gobnait Ní Bhruadair, Constance Markievicz, Dan Breen, Michael Comyn, Art O'Connor, and Mrs. O'Malley, the mother of Ernie O'Malley.

Following his retirement from politics, Tadhg lived for a time in Ballylanders, before moving in with his sister, Bridget O'Donnell, at her home in Scarteen.