<<the qeustion was not as to whether Ireland does or does not need an immigration policy
It was if an immigration policy is implicity descriminatory
(eventually, that is)>>
Given the complexity of present-day society and administration of state systems, laws and benefits an "immigration policy" is surely an essential part of state structure? Ernst Gellner is one author who tries to address ideas of state, nation group and nationalism in the context of culture and economics and suggests the world is moving towards a global, homogenous single state.........but we are not there yet!
Meanwhile states and union-states have (ideally!) consensual laws threshed out and administered by the legislature in the best perceived interests of "the people".......whoever that particular "people" may be and however they define themselves, inclusively or exclusively, temporarily or for all time.
To answer a question which addresses structure and process with "we're all from mixed stock ANYWAY back to the Vikings/Picts/Romans/Goths" perhaps spins the issue along lines of ethnicity and what is nowadays being carelessly termed "race".
An immigration policy IS INDEED intrinsically discriminatory but that does not mean it is racist or unfair. The problem lies with the word "discriminatory". To "discriminate" means to distinguish between, to choose. Legislation on who can come in the group reflects (a) the skills-needs of the group and (b) available resources. If there are plentiful resources historically anyone who chooses can join, contribute and benefit from membership of the "in"-group. HOWEVER when resources are scarce(er) as in the case of shortage of food, or housing, or benefits, or school-places, or medicine........then the group (for its survival!) can "choose" and "discriminate" to NOT be open to ALL newcomers as this puts undue pressure on those resources and can actually endanger the survival and consensuality of the group, and the wellbeing of its individual members........destroy "the group".
The furore that generally sets up when terms such as "immigration policy" are mentioned arises, I think, from the unfounded assumption that the "discrimination" around membership and resources must of necessity mean racial (or tribal!) prejudice against outsiders. Prejudice IS invidious and unacceptable......but prejudice is not the same as "discrimination". Discrimination on the basis of religion or skin colour is prejudice.
Policies limiting membership to various groups is endemic in contemporary social groups and is an historic reality. Group boundaries are created by difference between those "in" and those "out" and this "discrimination" (distinction between - NOT prejudice!) is not destructive or invidious, but prudent planning and administration. I happen to be white, female, English-speaking. Australian immigration policy would "discriminate" against me because (a) I do not hold qualifications in disciplines where they need incomers and (b) I am over 35. Similarly I would have difficulty getting a residence visa and a job in India. To describe the latter (but not the former, curiously!) event as "racial prejudice against whites enshrined in Indian immigration policy" would be to seriously misunderstood a (useful, consensual) discriminatory mechanism.