Ferguson settled in Dublin, where he practiced law. In
1846, he toured European
museums, libraries and
archaeological sites with strong connections to Irish scholarship. He married in
1848 while he was defending the
Young Irelander poet
Richard Dalton Williams.
As well as his poetry, Ferguson contributed a number of articles on topics of Irish interest to antiquarian journals. In
1863, he travelled in
Brittany, Ireland,
Wales, England and
Scotland to study
megaliths and other archaeological sites. These studies were important to his major antiquarian work,
Ogham Inscriptions in Ireland, Wales and Scotland, which was published posthumously in
1887.
His collected poems,
Lays of the Western Gael was published in
1865, resulting in the award of a
degree LL.D. honoris causa from Trinity. He wrote many of his poems with both Irish and English translations. In 1867, Ferguson retired from the bar to take up the newly created post of Deputy Keeper of the Public Records in Ireland. As reward for his services, he received a knighthood in
1878.
Ferguson's major work, the long poem
Congal was published in
1872 and a third volume,
Poems in
1880. In
1882, he was elected President of the
Royal Irish Academy, an organisation dedicated to the advancement of science, literature and antiquarian studies. He died in
Howth, just outside Dublin city, and was buried in
Donegore, County Antrim.