Daniel was born in Cork but lived most of his life in England where ehe made his living as an artist and illustrator. He illustrated several of Charles Dickens’ Christmas books for instance. In 1858 he was commissioned to make two paintings; one depicting Wellington, the second Nelson. He was chosen because of his picture in 1854 of the marriage of Strongbow to Aoife of Leinster. His works were huge and very detailed. His health suffered and he died on 25 April 1870.
Maclise would have been able to refer to Gerald of Wales for information about the wedding as it provides an account of the Siege of Waterford which took place before the wedding. To all intents and purposes the images looks like a monumental depiction in a neoclassical tradition using all of the theatricality which can be associated with Victorian history paintings. The light falls upon Aoife, an innocent, about to marry Strongbow fully armed and clad in black armour. There’s a reason why the old westerns placed white hats upon the goodies and black ones upon the heads of the baddies! it would be rather unexpected for a parliamentary painter descended from Scottish ancestors to start expressing Nationalist sympathies but there must be a reason why Strongbow also has his foot on top of a fallen Celtic cross.
For a closer look at the painting and to make your own mind up about whether Maclise was simply presenting us with a table from Irish history or using all the theatricality of his art to express sympathy for the Irish cause why not head across to the National Gallery of Ireland: https://www.nationalgallery.ie/explore-and-learn/conservation-and-research-projects/strongbow-aoife/symbols