Hull carded a four-under 69 at National Golf Club to finish four under par alongside European number one Carlota Ciganda from Spain.
The 17-year-old would have led on her own but for a bogey on the 18th, her sole dropped shot in a round also containing five birdies.
Hull only turned professional this year, but finished second on her Ladies European Tour debut in Morocco in March.
"It was a really good round but I'm very disappointed because I three-putted the last hole. I haven't parred that hole yet," Hull said. "It was a really good day but I'm a bit annoyed. You want to be as good as you can."
Ciganda carded three birdies and two bogeys in her 72, while overnight leaders Trish Johnson and Laura Davies slipped two and five shots off the lead with respective rounds of 75 and 78.
Holland's Christel Boeljon, seeking a third straight win in the event, carded a superb 67, the lowest round of the day, to lie one off the lead on three under.