The form of the reinforce around the trunnions precludes sighting. It is clearly an industrial age product, perhaps designed by patternmakers in a foundry but not by gunmakers, and the shapes suggest mid-later nineteenth century.
I feel that a line-throwing gun is a very good suggestion. There may be catalogue or other documentary evidence of its original purpose.
I too have an unresolved cannon ID of the period:
Evans and Lowe Cannon, Dundee