If this is suggesting loan deals, then the same current rules would apply, no short term deal would be permitted by the FAI and only players arriving in the transfer window could move to an Irish club, similar to say what Waterford have been doing recently.
This would mean a player having to be properly registered for a period from start of our season up to end of 30th June I think, or from after this period to the end of our season.
Technically a player in this instance could transfer from Bournemouth to Dundalk in January, but they would need to stay until 30 June, or until end of the season, if it was the latter, they would probably be ruled out of playing in UK until 1st Jan of the following year.