Horan had ended his romance with the Irish model and was recently been linked to singer Ellie Goulding, however, it's thought the 20-year-old star has secretly reunited with his former flame, reported Contactmusic.
"Niall wants everything kept completely under the radar as he feels they simply won't survive if they're being watched. But the truth is, he's mad about her and when he's in Ireland they're spending almost every second together," a source said.
Whelan has reportedly been jetting in from Ireland to meet Horan in recent months and she is friends with the hunk's bandmates, Zayn Malik, Liam Payne, Harry Styles and Louis Tomlinson.