how to find a father I have never known

Since your mother is dead, you may need to get creative. Are there old letters or diaries? What about friends or siblings of your mother who you could ask? What's your birth certificate say? You may very well need to hire a professional, as there are not a lot of leads, plus the search would likely have to be international in scope. But try these other methods first.

Also, you might want to get your DNA analyzed. Ancestry.com offers them (among many other providers). I cannot comment on the price, or the breadth, depth, and accuracy of the testing, but at least they don't appear to be scammers. I am suggesting this not only for your own peace of mind health-wise, but it might give you some clues if, say, you are a sickle cell anemia carrier (because that could mean that you have relatives with the full-blown disease), or the test percentages come up as overwhelmingly Irish, you may find you have a lot of white relatives.