Crossing Irish borders

[owsiddiqui]

I have been through to the post and planning to move to Ireland with my EEA sister as dependent. I was wondering if somebody could tell or guide me about travelling to Ireland from Northern Ireland with my sister. I know travelling with EEA dependent as "other family member" does not give automatic right on the directive but clause 4 of article 5 gives the right to get necessary time to obtain necessary documents within a reasonable period of time. Does this mean that the official could give entry visa (i.e. short stay C visa) at the time of travel rather than getting a visa before travelling with the EEA national? Or do I need to obtain a short stay visa before travelling to Ireland? Any help would be much appreciated as soon as possible.
Also please find the Article 5 of the Directive 2004/38/EC

Article 5 Clause 4
4. Where a Union citizen, or a family member who is not a national of a Member State, does not have the necessary travel documents or, if required, the necessary visas, the Member State concerned shall, before turning them back, give such persons every reasonable opportunity to obtain the necessary documents or have them brought to them within a reasonable period of time or to corroborate or prove by other means that they are covered by the right of free movement and residence.