眠り込んでしまった

fell fast asleep

→ 眠り込む
phrase Rare #38,294
fell fast asleep 眠り込む means to fall deeply asleep or doze off. しまった emphasizes that it happened completely or unintentionally, often with a sense of regret.
JA: 本を読んでいたら、いつの間にか眠り込んでしまった。
EN: I was reading a book and ended up falling fast asleep before I knew it.

トム起きていようとしたのだが、すぐに眠り込んでしまった

Tom tried to stay awake, but he quickly fell asleep.

座るや否や眠り込んでしまった

No sooner had I sat down than I fell fast asleep.

疲れていたので眠り込んでしまった

I was tired, so I fell asleep.

彼はテレビのスイッチを入れてテレビの前に座ったまますぐに眠り込んでしまった

He turned on the TV and immediately fell asleep sitting in front of it.