The 29-year-old said he made his decision to stay at Toulouse despite knowing it would put him out of contention for an England spot.
“You know what you sacrifice,” Willis said of his Test career.
“It’s something I still want to do, but rules are rules.”
Willis is now, in theory, available for selection by Ireland.
Three years have passed since he last appeared for England and, under World Rugby’s ‘birthright’ rule, he could now represent a country which he, his parents or his grandparents were born in. Willis’ grandfather was born in Belfast.
Ireland’s strong preference for home-based players has been relaxed on rare occasions in the past with Jonny Sexton and Tommy Bowe continuing to play for their country despite moves to Racing 92 and the Ospreys respectively.
However, the likes of Simon Zebo have also found themselves excluded after heading overseas.
It is highly unlikely any compromise would be found that allows Willis to play for Ireland while remaining with Toulouse.
“I’m a quarter Irish, but they have the same rules as England,” said Willis.
“You need to do some digging, that’s your job, you find the way [around the rules]!”