According to Cassidy, the word ‘block’, in the sense of a city block, derives from the Irish bealach, meaning road or way. This is a typical Cassidy claim, in that it is completely unrealistic and presents no evidence beyond a vague phonetic similarity. A very vague similarity. Bealach is usually pronounced with the final ch either silent or like a breathy h, though you could choose to pronounce it more strongly, as in the ch sound of Scottish loch. However, it would never be pronounced ballack, let alone block, so it is unlikely as an origin for block and in any case, it refers to the road, not to the space between roads.
Another point to remember is that block is a perfectly sensible and long-established word in English to describe the parcels or areas of land in cities which were created along with the grid system in the new world. Just as a person looking at a stone wall will see the blocks of stone laid out in a pattern, the mapmaker will see blocks of buildings with roads between. The word block in the sense of a lump of stone or wood is an ancient one in English, dating back at least to the 13th century. Here’s a brief account of the word’s origins from etymonline:
“solid piece,” c.1300, from O.Fr. bloc “log, block” (13c.), via M.Du. bloc “trunk of a tree” or O.H.G. bloh, both from PIE *bhlugo-, from *bhelg- “a thick plank, beam” (see balk). Slang sense of “head” is from 1630s. The meaning in city block is 1796, from the notion of a “compact mass” of buildings; …
Incidentally, bloc is also a fairly common and long-established loanword in Irish, where Bloc na Nollag (the Christmas Block) was the traditional Yule Log.
The grid system was first laid out in Cassidy’s home city of New York by the 1811 Commissioners’ Plan for New York. Let’s hear from the Commissioners themselves:
‘The northerly side of number one begins at the southern end of Avenue B and terminates in the Bower lane; number one hundred and fifty-five runs from Bussing’s Point to Hudson river, and is the most northern of those which is was thought at all needful to lay out as part of the city of New York, excepting the Tenth avenue, which is continued to Harlem river and strikes it near Kingsbridge. These streets are all sixty feet wide except fifteen, which are one hundred feet wide, viz.: Numbers fourteen, twenty-three, thirty-four, forty-two, fifty-seven, seventy-two, seventy-nine, eighty-six, ninety-six, one hundred and six, one hundred and sixteen, one hundred and twenty-five, one hundred and thirty-five, one hundred and forty-five, and one hundred and fifty-five–the block or space between them being in general about two hundred feet’.
In other words, the absurd idea that ‘city block’ came from the Irish language simply ignores all the evidence.