Trade unions may, for the purpose of analysis, be divided into a number of categories, as follows:

Craft union : The earliest form of trade union , one whose membership is restricted to a particular category of skilled or craft workers , for instance, printers, carpenters and so on. Membership of craft unions is usually restricted to those who have completed the recognised qualification for the craft or skill in question, frequently an apprenticeship. Being closed unions (see below), craft unions may be quite small: for example, the National Union of Sheet Metal Workers of Ireland has 720 members as of January 1992, and the Cork Operative Butchers' Society 149 members at the same date.

General union : A trade union which has no form of restriction on categories of worker who may join. There are no limits on the industrial or occupational area of recruitment. As such, general unions are open unions (see below), and tend to be fairly large. The largest general union in Ireland, the Services, Industrial, Professional and Technical Union (SIPTU), has 190,500 members in the Republic of Ireland as of December 1992 and amounts to 43 per cent. of the Republic's total ICTUaffiliated trade union membership. As some trade unions move away from their original areas of recruitment, so they may become more like general unions; for example, the British-based union EETPU (now amalgamated with the engineering union AEU to become AEEU) which was originally a craft union, has now moved into a number of other sectors and recruited many semiskilled workers, thus having become closer to a general union than a craft union. Similarly, the retail and distributive union IDATU has widened its areas of recruitment.

Industry union : A union which confines its recruitment to one particular industry or industrial sector, and aims to recruit all workers at all grades in that area. There are very few "pure" industrial unions in Ireland, but the Communications Workers' Union is the most prominent union in the postal and telecommunications sector, and the non-ICTU National Busworkers' Union competes with SIPTU in recruiting workers in the two CIE bus companies. In attempting to recruit all grades of workers in the industry, industry unions are therefore vertical unions; however, the existence of the Communication Managers' Union blocks the CWU's attempts to recruit managerial staff.

Occupational union : A union which confines its recruitment to particular occupational areas, many of which may require the holding of certain qualifications in the same way as craft unions. Occupational unions mainly exist in the white-collar area: for teachers, civil servants, police, bank employees and so on. Many Irish occupational unions are structured along the lines of grade or type of staff: for example, membership of civil service unions is highly structured according to grade, and the four teaching unions respectively represent primary, secondary, vocational and university teachers. Occupational unions also exist for workers in the health services, but these are more commonly referred to as professional unions.

Professional unions : Unions whose membership is restricted to members of the same or similar professional areas. Professional unions are thus another form of craft union, but members of these unions would regard themselves as being of a higher status. Examples of professional unions are the National Union of Journalists and unions representing health professionals, such as those for doctors or nurses, and teaching unions. Most health unions are not affiliated to the ICTU, with the exception of the Irish Nurses' Organisation which affiliated in 1990.

Vertical union : A union which recruits vertically through grades; in other words, workers from the lowest "shop-floor" grade up to management are recruited. Many professional unions are vertical: for example, those representing nurses and teachers, among others. The difficulty with vertical unions is the potential they contain for conflict between the grades, which may be manifested in different views on what union policy should be in a particular area, or even in cases where the management structure means that the workers in dispute and the managers against whom the dispute is taken are in the same union.

White-collar union : A union which recruits primarily whitecollar workers, in other words, non-manual workers. Whitecollar unions represent general clerical workers and workers from the services sector, including financial services. The main expansion in trade union membership since the 1960s has been in this area. The largest white-collar union is the MSF, although the general unions have a large proportion of white-collar workers in membership, and trade unions representing clerical grades in the civil service and local government may also be classed as whitecollar.

Many writers feel that the approach to union structure using the categories listed above now has limited usefulness, since many unions may come under more than one category and in any case, the boundaries between the categories are vague. A simpler classification is the distinction between open and closed unions. An open union is one which imposes no restrictions on its area of organisation (except, perhaps, those imposed by the ICTU), and will recruit all workers in the occupations where the employers operate. A closed union, on the other hand, restricts its membership to particular categories: members of a particular trade or profession, or employees of a particular employer or in a particular industry. The aims of the two categories of union are very different: an open union is interested in increasing recruitment simply to increase its numerical strength, while a closed union aims to control the supply of labour to a particular industry or occupation, and other than this has no interest inincreasing numbers as such. These categories of trade union are not fixed: closed unions may become open as the industries in which they operate change, or as their policy changes.

Please note: the European industrial relations glossaries were compiled between 1991 and 2003 and are not updated. For current material see the European industrial relations dictionary.