This is another stupid and unfounded claim made by Cassidy in How The Irish Invented Slang. Masher is a slang term for a young man of fashion who frequented 19th century theatres because of his devotion to the leading ladies.
There is a discussion of its origins here in an excellent blog post from Anatoly Liberman: http://blog.oup.com/2011/01/masher/.
And here’s another from World Wide Words: http://www.worldwidewords.org/qa/qa-mas1.html
Both of these sources are inclined to regard masher and mash as being extensions of the English word mash meaning to crush and both of them point to the similarity between the uses of mash and the uses of the word crush.
Cassidy’s claim was that it derives from the Irish maiseach, an obscure adjective meaning beautiful or elegant (according to Ó Dónaill’s dictionary, maisiúil is the usual adjective formed from maise. I wouldn’t use maisiúil or maiseach, though I use maise all the time). How an adjective meaning beautiful in Irish gave rise to a noun meaning lady’s man and a verb meaning to have a crush in English is not explained, but then Cassidy probably didn’t know what an adjective or a verb are, and he certainly had very little in the way of common sense or academic ability.
In short, this is yet another completely ridiculous claim, unsupported by any evidence at all.