Late enough that the position and its start date have settled, and early enough that a refusal would still leave room to correct the offer and try again. Admission is granted for a period not exceeding three years and is tied to the employment described, so applying against a description still in motion is the real hazard.
Hospitality timetables and immigration timetables run at different speeds
Seasonal employers think in opening dates, and that pressure produces applications made before a job description has stopped moving. Imelda should treat the offer as final only once three things hold still: what she will be responsible for, the day she starts, and how long the resort expects the arrangement to run. If the properties open in May, working backwards through the employer's drafting time, the assembly of letters and transcripts, and a margin for one correction, points at beginning the exercise in winter rather than spring.
She should also avoid two improvisations that a compressed timetable encourages. Managing the American properties remotely from Victoria County before admission raises awkward questions about when the employment actually started. Travelling in advance as a visitor and then performing productive work is a separate difficulty again, and neither should be attempted without advice.