St Seiriol's church was located within an irregular-shaped churchyard on the north-east side of Thomas Street, on the west side of Holyhead. It was built in 1853-4 to designs of Charles Verelst, architect, of Liverpool. Images taken before its demolition suggest its plan comprised nave, apsidal chancel, a lean-to along the south lateral wall with gabled porch, and an imposing tower on the west.
The spire once dominated the skyline of Holyhead but the church was found to be structurally unsafe in the 1980s, fund-raising efforts for repairs failed, and it was finally demolished in 1992. The churchyard cemetery survives.

Sources:
http://www.anglesey.info/holyhead-st-seiriols-church.htm
http://commons.wikimedia.org/wiki/File:St_Seiriol's_Church_Holyhead_1853.jpg
Detailed plan available (churchplansonline.org)

David Leighton, RCAHMW, 11 November 2014