There are multiple pages of news articles about the Emmet Guards and William Regan. Worcester Daily Spy has an article Tuesday, May 29, 1894 announcing his retirement.
You may know this, but I did see that he married Margaret Carr on Feb 24, 1881 - saw this on ancestry - could not find it on Family Search. His parents: Michael and Nora. Hers: Peter and Margaret. D. H. O'Neill officiating. Also, only Margaret Carr born in Worcester to Peter: 15 Mar 1853 Peter, Cath per ancestry.com
Could your William Regan be the brother of Michael and Hanora Regan who lived in Milford, Ma? They had son David Regan 18 Nov 1875 in Milford. Wife was Hanora Kirby. From Family Search.
Per 1870 census:
Michael Regan the father was born in Ireland in 1843.
Other children born in Ireland:
Mary E Reagan 1 (b. in Mass)
James Reagan 8/12
big gap?
William Reagan 16
Thomas Reagan 22
Hanora died in Milford 12 Dec 1878.
Worcester in the Spanish AMerican War:
http://goo.gl/d1QTYNo information so far about DOD for either.