He went on to study at Trinity College Dublin, where in 1930 he was elected a Scholar and graduated with a BA in classics in 1932 and a Bachelor of Divinity degree in 1936.
He was ordained a deacon in 1934 and a priest in 1936, beginning his ministry as a curate at St Bartholomew's, Clyde Road, Dublin under Canon W.C.
During this time, he maintained a courteous relationship with John Charles McQuaid, his Roman Catholic counterpart as Archbishop of Dublin.
Alongside Cardinal William Conway, Simms chaired the first official ecumenical meeting between the leaders of Ireland's Protestant Churches and the Catholic Church in Ballymascanlon Hotel, Dundalk, County Louth on 26 September 1973, an important meeting amidst the increasing violence in Northern Ireland.
His weekly Thinking Aloud column in the Irish Times was a popular reflection, and ran continuously for thirty-eight years.