八辻が原(kanji 八, not katakana ハ) is the name of a location, i.e., 先 is not a part of a name.
も doesn't mean "too/also" there. もう十五年も shows that 15 years is long-term "already 15 years long".
八辻が原 is read やつじがはら.
先 means that 十軒長屋 is over/beyond/across 八辻が原 (from the viewpoint of the central Edo).
さき【先】
6 〔前方〕
その店はこの先50メートルほどのところにある
You will find the store about fifty meters ahead.
彼は私より20メートル先を走っていた
He was running twenty meters ahead of me.
パン屋はちょっと先です
The bakery is right up there.
銀行の少し先で降ろしてくれ
Drop me off just beyond [past] the bank.
新幹線で京都より先に行ったことがない
I've never taken the Shinkansen further [farther] than Kyoto.